[clang] [MinGW] Add the compiler rt libdirs to the search path

[mstorsjo]

Summary

This matches what is done for MSVC in b8000c0ce84541c5b5535419234fb65ce77d6756. Since that commit, compiler rt sanitizer libraries aren't linked to with absolute path on windows, but using their basenames, requiring the libdirs to be passed to the linker.

This fixes undefined behaviour sanitizer on MinGW after b8000c0ce84541c5b5535419234fb65ce77d6756.

Not sure what would be a good test for this, as the lib/clang/<version>/lib/windows directory doesn't exist, it won't show up in tests...

[rnk]

lgtm

clang/lib/Driver/ToolChains/MinGW.cpp

 #include "clang/Driver/Compilation.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
 #include "clang/Driver/SanitizerArgs.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/VirtualFileSystem.h"
 #include <system_error>
 
 using namespace clang::diag;
 using namespace clang::driver;
 using namespace clang;
 using namespace llvm::opt;
 
 /// MinGW Tools
if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
     }
     if (Args.hasArg(options::OPT_pg))
       CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("gcrt2.o")));
     CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crtbegin.o")));
   }
 
   Args.AddAllArgs(CmdArgs, options::OPT_L);
   TC.AddFilePathLibArgs(Args, CmdArgs);
+
+  // Add the compiler-rt library directories if they exist to help
+  // the linker find the various sanitizer, builtin, and profiling runtimes.
+  for (const auto &LibPath : TC.getLibraryPaths()) {
+    if (TC.getVFS().exists(LibPath))
+      CmdArgs.push_back(Args.MakeArgString("-L" + LibPath));
+  }
+  auto CRTPath = TC.getCompilerRTPath();
+  if (TC.getVFS().exists(CRTPath))
+    CmdArgs.push_back(Args.MakeArgString("-L" + CRTPath));
+
   AddLinkerInputs(TC, Inputs, Args, CmdArgs, JA);
 
   // TODO: Add profile stuff here
 
   if (TC.ShouldLinkCXXStdlib(Args)) {
     bool OnlyLibstdcxxStatic = Args.hasArg(options::OPT_static_libstdcxx) &&
                                !Args.hasArg(options::OPT_static);
     if (OnlyLibstdcxxStatic)