[LLD] [COFF] Allow invoking lib.exe mode via -lib in addition to /lib

[mstorsjo]

Fixed an issue with a test that only manifests if the test dir is cleaned before running the tests, that had a stray "-lib" argument that only caused warnings before, but now caused the tests to break.

[mstorsjo]

Reuploading to retrigger CI.

[rnk]

lgtm

lld/COFF/Driver.cpp

   InitializeAllTargetInfos();
   InitializeAllTargets();
   InitializeAllTargetMCs();
   InitializeAllAsmParsers();
   InitializeAllAsmPrinters();
 
   // If the first command line argument is "/lib", link.exe acts like lib.exe.
   // We call our own implementation of lib.exe that understands bitcode files.
-  if (argsArr.size() > 1 && StringRef(argsArr[1]).equals_lower("/lib")) {
+  if (argsArr.size() > 1 && (StringRef(argsArr[1]).equals_lower("/lib") ||
+                             StringRef(argsArr[1]).equals_lower("-lib"))) {
     if (llvm::libDriverMain(argsArr.slice(1)) != 0)
       fatal("lib failed");
     return;
   }
 
   // Parse command line options.
   ArgParser parser;
   opt::InputArgList args = parser.parse(argsArr);

lld/test/COFF/driver.test

 # RUN: not lld-link nosuchfile.obj >& %t.log
 # RUN: FileCheck -DMSG=%errc_ENOENT -check-prefix=MISSING %s < %t.log
 MISSING: 'nosuchfile.obj': [[MSG]]
 
 # RUN: lld-link --version | FileCheck -check-prefix=VERSION %s
 VERSION: {{LLD [0-9]+\.[0-9]+}}
 
 # RUN: yaml2obj %p/Inputs/export.yaml -o %t.obj
 # RUN: lld-link /out:%t.dll /dll %t.obj
 # RUN: not lld-link /out:%t.exe %t.dll 2>&1 | FileCheck -check-prefix=BADFILE %s
 BADFILE: bad file type. Did you specify a DLL instead of an import library?
 
 # RUN: lld-link /lib /help | FileCheck -check-prefix=LIBHELP %s
+# RUN: lld-link -lib -help | FileCheck -check-prefix=LIBHELP %s
 LIBHELP: OVERVIEW: LLVM Lib
 
 # RUN: env LLD_IN_TEST=1 not lld-link /WX /lib 2>&1 | FileCheck -check-prefix=LIBBAD %s
 LIBBAD: ignoring /lib since it's not the first argument
 
 # RUN: yaml2obj %p/Inputs/hello32.yaml -o %t.obj
 # RUN: not lld-link /out:/ %t.obj 2>&1 | FileCheck -check-prefix=DIR %s
 DIR: cannot open output file