[LLD] [MinGW] Look for libs named "<libname>.lib" even with -static

This matches how ld.bfd works in practice; this fixes
mstorsjo/llvm-mingw#305.

Adding a test for the new lib name combination that this allows, but
also adding a few negative tests for combinations that aren't
matched when -static is specified (because this change in itself
didn't break any of the existing tests either).

The logic in how ld.bfd looks for various libraries based on
an -l<libname> argument is rather complex; the
ldemul_open_dynamic_archive function looks for various combinations:
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/emultempl/pep.em;h=e68d1e69f17ad73af065b6bed19ae89ded913172;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l2066
This function is only called if looking for dynamic libraries
(i.e. if -static wasn't specified):
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/ldfile.c;h=731ae5f7aedcf921bd36a1b32a3e0f5bfa189071;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l365

However even this function is skipped, it still looks for libraries
in the form of "lib<libname>.a" (this is what lld did before):
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/ldfile.c;h=731ae5f7aedcf921bd36a1b32a3e0f5bfa189071;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l440
But it also calls a format specific function called
ldemul_find_potential_libraries, which for PE targets looks for
files named "<libname>.lib":
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/emultempl/pep.em;h=e68d1e69f17ad73af065b6bed19ae89ded913172;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l2175

Differential Revision: https://reviews.llvm.org/D135651

lld/MinGW/Driver.cpp

@@ -139,9 +139,9 @@ searchLibrary(StringRef name, ArrayRef<StringRef> searchPaths, bool bStatic) {
     }
     if (Optional<std::string> s = findFile(dir, "lib" + name + ".a"))
       return *s;
+    if (Optional<std::string> s = findFile(dir, name + ".lib"))
+       return *s;
     if (!bStatic) {
-      if (Optional<std::string> s = findFile(dir, name + ".lib"))
-        return *s;
       if (Optional<std::string> s = findFile(dir, "lib" + name + ".dll"))
         return *s;
       if (Optional<std::string> s = findFile(dir, name + ".dll"))

lld/test/MinGW/lib.test

@@ -38,9 +38,20 @@ RUN: ld.lld -### -m i386pep -L%t/lib -lnoprefix -lmsvcstyle 2>&1 | FileCheck -ch
 OTHERSTYLES: noprefix.dll.a
 OTHERSTYLES-SAME: msvcstyle.lib
 
+RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoprefix 2>&1 | FileCheck -check-prefix=ERROR-NOPREFIX %s
+ERROR-NOPREFIX: unable to find library -lnoprefix
+
+RUN: ld.lld -### -m i386pep -L%t/lib -static -lmsvcstyle 2>&1 | FileCheck -check-prefix=MSVCSTYLE %s
+MSVCSTYLE: msvcstyle.lib
+
 RUN: echo > %t/lib/libnoimplib.dll
 RUN: echo > %t/lib/noprefix_noimplib.dll
 RUN: ld.lld -### -m i386pep -L%t/lib -lnoimplib 2>&1 | FileCheck -check-prefix=DLL1 %s
 RUN: ld.lld -### -m i386pep -L%t/lib -lnoprefix_noimplib 2>&1 | FileCheck -check-prefix=DLL2 %s
 DLL1: libnoimplib.dll
 DLL2: noprefix_noimplib.dll
+
+RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoimplib 2>&1 | FileCheck -check-prefix=ERROR-NOIMPLIB %s
+RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoprefix_noimplib 2>&1 | FileCheck -check-prefix=ERROR-NOPREFIX-NOIMPLIB %s
+ERROR-NOIMPLIB: unable to find library -lnoimplib
+ERROR-NOPREFIX-NOIMPLIB: unable to find library -lnoprefix_noimplib