[LLD] [MinGW] Recognize the -rpath option (llvm#102886)

GNU ld silently accepts the -rpath option for Windows targets, as a
no-op.

This has lead to some build systems (and users) passing this option
while building for Windows/MinGW, even if Windows doesn't have any
concept like rpath.

Older versions of Conan did include -rpath in the pkg-config files it
generated, see e.g.

https://github.com/conan-io/conan/blob/17c58f0c61931f9de218ac571cd97a8e0befa68e/conans/client/generators/pkg_config.py#L104-L114
and
https://github.com/conan-io/conan/blob/17c58f0c61931f9de218ac571cd97a8e0befa68e/conans/client/build/compiler_flags.py#L26-L34
- and see mstorsjo/llvm-mingw#300 for user
reports about this issue.

Recognize the option in LLD for MinGW targets, to improve drop-in
compatibility compared to GNU ld, but produce a warning to alert users
that the option really has no effect for these targets.

lld/MinGW/Driver.cpp

@@ -448,6 +448,9 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
       add("-errorlimit:" + s);
   }
 
+  if (auto *a = args.getLastArg(OPT_rpath))
+    warn("parameter " + a->getSpelling() + " has no effect on PE/COFF targets");
+
   for (auto *a : args.filtered(OPT_mllvm))
     add("-mllvm:" + StringRef(a->getValue()));
 

lld/MinGW/Options.td

@@ -243,6 +243,9 @@ defm: EqNoHelp<"sysroot">;
 def: F<"sort-common">;
 def: F<"start-group">;
 
+// Ignored options, that produce warnings
+defm rpath: EqNoHelp<"rpath">;
+
 // Ignore GCC collect2 LTO plugin related options. Note that we don't support
 // GCC LTO, but GCC collect2 passes these options even in non-LTO mode.
 def: J<"plugin-opt=-fresolution=">;

lld/test/MinGW/driver.test

@@ -446,3 +446,9 @@ RUN: ld.lld -### foo.o -m i386pep --build-id=none 2>&1 | FileCheck -check-prefix
 RUN: ld.lld -### foo.o -m i386pep -s 2>&1 | FileCheck -check-prefix=NO_BUILD_ID %s
 RUN: ld.lld -### foo.o -m i386pep -S 2>&1 | FileCheck -check-prefix=NO_BUILD_ID %s
 NO_BUILD_ID: -build-id:no
+
+RUN: ld.lld -### foo.o -m i386pep -rpath foo 2>&1 | FileCheck -check-prefix=WARN_RPATH %s
+RUN: ld.lld -### foo.o -m i386pep --rpath foo 2>&1 | FileCheck -check-prefix=WARN_RPATH %s
+RUN: ld.lld -### foo.o -m i386pep -rpath=foo 2>&1 | FileCheck -check-prefix=WARN_RPATH %s
+RUN: ld.lld -### foo.o -m i386pep --rpath=foo 2>&1 | FileCheck -check-prefix=WARN_RPATH %s
+WARN_RPATH: warning: parameter -{{-?}}rpath has no effect on PE/COFF targets