[lld] Add (ignored) /link flag to lld-link for compatibility with MSVC link.exe (#168364)

Various build tools may produce command lines invoking clang-cl and
lld-link which contain /link twice like so: e.g. `clang-cl.exe
sanitycheckcpp.cc /Fesanitycheckcpp.exe .... /link /link ...`

If link.exe is used, it ignores the extra `/link` and just issues a
warning, however lld-link tries to treat `/link` as a file name.

This PR adds a flag which is ignored in order to improve compatibility
with link.exe

There's some extra context including an "in-the-wild" example and
reproducer of the problem here:
https://github.com/frankier/meson_clang_win_activation

Co-authored-by: Frankie Robertson <[email protected]>

Author: frankier

lld/COFF/DriverUtils.cpp

@@ -862,6 +862,9 @@ opt::InputArgList ArgParser::parse(ArrayRef<const char *> argv) {
                 << "', did you mean '" << nearest << "'";
   }
 
+  if (args.hasArg(OPT_link))
+    Warn(ctx) << "ignoring /link, did you pass it multiple times?";
+
   if (args.hasArg(OPT_lib))
     Warn(ctx) << "ignoring /lib since it's not the first argument";
 

lld/COFF/Options.td

@@ -71,6 +71,7 @@ def noimplib : F<"noimplib">,
 def lib : F<"lib">,
     HelpText<"Act like lib.exe; must be first argument if present">;
 def libpath : P<"libpath", "Additional library search path">;
+def link : F<"link">, HelpText<"Ignored for compatibility">;
 def linkrepro : Joined<["/", "-", "/?", "-?"], "linkrepro:">,
     MetaVarName<"directory">,
     HelpText<"Write repro.tar containing inputs and command to reproduce link">;

lld/test/COFF/driver.test

@@ -17,6 +17,9 @@ 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: env LLD_IN_TEST=1 not lld-link /link 2>&1 | FileCheck -check-prefix=LINKBAD %s
+LINKBAD: ignoring /link, did you pass it multiple times?
+
 # 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