[Clang] Remove support for legacy pass manager

This removes the -flegacy-pass-manager and
-fno-experimental-new-pass-manager options, and the corresponding
support code in BackendUtil. The -fno-legacy-pass-manager and
-fexperimental-new-pass-manager options are retained as no-ops.

Differential Revision: https://reviews.llvm.org/D123609

clang/include/clang/Basic/CodeGenOptions.def

@@ -65,7 +65,6 @@ CODEGENOPT(DisableO0ImplyOptNone , 1, 0) ///< Don't annonate function with optno
 CODEGENOPT(ExperimentalStrictFloatingPoint, 1, 0) ///< Enables the new, experimental
                                                   ///< strict floating point.
 CODEGENOPT(EnableNoundefAttrs, 1, 0) ///< Enable emitting `noundef` attributes on IR call arguments and return values
-CODEGENOPT(LegacyPassManager, 1, 0) ///< Use the legacy pass manager.
 CODEGENOPT(DebugPassManager, 1, 0) ///< Prints debug information for the new
                                    ///< pass manager.
 CODEGENOPT(DisableRedZone    , 1, 0) ///< Set when -mno-red-zone is enabled.

clang/include/clang/Driver/Options.td

@@ -1935,15 +1935,10 @@ def fglobal_isel : Flag<["-"], "fglobal-isel">, Group<f_clang_Group>,
   HelpText<"Enables the global instruction selector">;
 def fexperimental_isel : Flag<["-"], "fexperimental-isel">, Group<f_clang_Group>,
   Alias<fglobal_isel>;
-defm legacy_pass_manager : BoolOption<"f", "legacy-pass-manager",
-  CodeGenOpts<"LegacyPassManager">, DefaultFalse,
-  PosFlag<SetTrue>,
-  NegFlag<SetFalse>,
-  BothFlags<[CC1Option]>>, Group<f_clang_Group>;
+def fno_legacy_pass_manager : Flag<["-"], "fno-legacy-pass-manager">,
+  Group<f_clang_Group>, Flags<[CC1Option, NoArgumentUnused]>;
 def fexperimental_new_pass_manager : Flag<["-"], "fexperimental-new-pass-manager">,
   Group<f_clang_Group>, Flags<[CC1Option]>, Alias<fno_legacy_pass_manager>;
-def fno_experimental_new_pass_manager : Flag<["-"], "fno-experimental-new-pass-manager">,
-  Group<f_clang_Group>, Flags<[CC1Option,NoDriverOption]>, Alias<flegacy_pass_manager>;
 def fexperimental_strict_floating_point : Flag<["-"], "fexperimental-strict-floating-point">,
   Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Enables experimental strict floating point in LLVM.">,