add Aro as a C frontend

[andrewrk]

Problems to solve in this branch before merging:

• Liveness and AIR printing want a ZIR instance. This issue is
  addressed by make AIR instruction for assembly not reference ZIR #10784.
• linker: updateFunc / updateDecl want a Module instance which
  currently represents only Zig code. These functions should be changed
  to accept a Compilation, not a Module. The relevant state that it
  needed to touch on Module should be moved to Compilation instead.
• Decl needs to be shared by both the C frontend and the Zig
  frontend. It currently has Zig-only fields and the functions to
  create Decl objects don't really fit the API that the C frontend
  needs.
• A lot of the incremental compilation infrastructure doesn't quite
  match up, for example Decl objects being owned by Namespaces which is
  a Zig concept. Maybe there could be 1 Namespace globally across all C
  files and 1 more Namespace per C file for functions and globals
  declared static.
• There is common code to be extracted from Sema that is shared with
  Aro frontend. All the helper functions having to do with the
  air_instructions and air_extra arrays.

There are many instances of calling @panic("TODO") that need to be
cleaned up.

There is a lot of compiler options that Zig is not communicating to Aro,
for example we do not pass the -D switches that we pass to Clang yet.
Need to audit addCCArgs and do the equivalent things for setting up
the Aro Compilation.

The Aro code is copied from https://github.com/Vexu/arocc, commit
7a0e54227e1ea2b2df8aa7bd2b7075f735109317. The only patches are to delete
Codegen and replace it with our own, and to delete main.zig, and add
Type and Value to lib.zig.

I believe the current plan is for main Aro development to happen
upstream on the arocc repository and periodically sync it downstream
with the Zig repository. It's up to @Vexu whether to keep that process
or change it in the future.

[andrewrk]

Current status on hello world:

$ ./zig-out/bin/zig build-exe hello.c -lc empty.zig --verbose-air
# Begin Function AIR: main:
# Total AIR+Liveness bytes: 257B
# AIR Instructions:         9 (81B)
# AIR Extra Data:           6 (24B)
# AIR Values Bytes:         4 (32B)
# Liveness tomb_bits:       8B
# Liveness Extra Data:      0 (0B)
# Liveness special table:   0 (0B)
  %0 = const_ty(*const fn([*c]i8, ...) callconv(.C) c_int)
  %1 = constant(*const fn([*c]i8, ...) callconv(.C) c_int, (decl ref))
  %2 = const_ty([15]u8)
  %3 = constant([15]u8, "Hello, World!\n\x00")
  %4 = const_ty(*const [15:0]u8)
  %5 = constant(*const [15:0]u8, (decl ref))
  %7 = constant(c_int, 0)

  %6!= call(%1!, [%5!])
  %8!= ret(%7!)
# End Function AIR: main

thread 489784 panic: Segmentation fault at address 0x8
/home/andy/dev/zig/src/Module.zig:751:34: 0x3faba9 in Module.Decl.getFileScope (zig)
        return decl.src_namespace.file_scope;
                                 ^
/home/andy/dev/zig/src/Module.zig:614:44: 0x59e852 in Module.Decl.nodeOffsetSrcLoc (zig)
            .file_scope = decl.getFileScope(),
                                           ^
/home/andy/dev/zig/src/Module.zig:609:37: 0x59a205 in Module.Decl.srcLoc (zig)
        return decl.nodeOffsetSrcLoc(0);
                                    ^
/home/andy/dev/zig/src/link/Elf.zig:2690:69: 0x706c57 in link.Elf.updateFunc (zig)
    const res = try codegen.generateFunction(&self.base, decl.srcLoc(), func, air, liveness, &code_buffer, .{
                                                                    ^
/home/andy/dev/zig/src/link.zig:453:77: 0x6fde29 in link.File.updateFunc (zig)
            .elf   => return @fieldParentPtr(Elf,   "base", base).updateFunc(module, func, air, liveness),
                                                                            ^
/home/andy/dev/zig/src/aro/Codegen.zig:314:26: 0x739e16 in aro.Codegen.genFn (zig)
    c.bin_file.updateFunc(mod, module_fn, air, liveness) catch |err| switch (err) {
                         ^
/home/andy/dev/zig/src/aro/Codegen.zig:84:27: 0x7108b1 in aro.Codegen.generateTree (zig)
            => try c.genFn(decl_node),
                          ^
/home/andy/dev/zig/src/Compilation.zig:3222:33: 0x6fea91 in Compilation.compileWithAro (zig)
    try aro.Codegen.generateTree(comp, &aro_comp, tree, arena);
                                ^
/home/andy/dev/zig/src/Compilation.zig:2832:32: 0x58df44 in Compilation.processOneJob (zig)
            comp.compileWithAro(c_source_file) catch |err| switch (err) {
                               ^
/home/andy/dev/zig/src/Compilation.zig:2691:30: 0x57a716 in Compilation.performAllTheWork (zig)
            try processOneJob(self, work_item, main_progress_node);
                             ^

I'm starting to wonder... should we perhaps lower to ZIR instead of AIR? The integration would be a lot less messy, and it's much more obvious how to parallelize it. One .c file would produce exactly one corresponding ZIR file. 🤔 🤔 🤔

[andrewrk]

Keeping the branch alive but closing the PR as I don't intend to merge this any time soon.