clang-cl: Fix path-based MSVC version detection

[hans]

Summary

The code wasn't taking the architecture-specific subdirectory into account.

[hans]

Nico asked about tests. This whole thing landed (r297851) without tests, and I'm not sure it's worth the effort adding one for this :-/

[rnk]

I'd rather not hardcode all the msvc archs. IMO we should just look up one level and check if that's called bin, and continue if so. I think I had:

bool IsBin = llvm::sys::path::filename(PathEntry).compare_lower("bin");
if (!IsBin) {
  PathEntry = llvm::sys::path::parent_path(PathEntry);
  IsBin = llvm::sys::path::filename(TestPath).compare_lower("bin");
}
if (IsBin) {
  ...

The compare_lower is also a fix, because my path looks like ".../VC/Bin".

[hans]

Addressing comments.

[rnk]

lgtm

lib/Driver/ToolChains/MSVC.cpp

for (llvm::StringRef PathEntry : PathEntries) {
       // cl.exe existing isn't a conclusive test for a VC toolchain; clang also
       // has a cl.exe. So let's check for link.exe too.
       ExeTestPath = PathEntry;
       llvm::sys::path::append(ExeTestPath, "link.exe");
       if (!llvm::sys::fs::exists(ExeTestPath))
         continue;
 
       // whatever/VC/bin --> old toolchain, VC dir is toolchain dir.
-      if (llvm::sys::path::filename(PathEntry) == "bin") {
-        llvm::StringRef ParentPath = llvm::sys::path::parent_path(PathEntry);
+      llvm::StringRef TestPath = PathEntry;
+      bool IsBin = llvm::sys::path::filename(TestPath).equals_lower("bin");
+      if (!IsBin) {
+        // Strip any architecture subdir like "amd64".
+        TestPath = llvm::sys::path::parent_path(TestPath);
+        IsBin = llvm::sys::path::filename(TestPath).equals_lower("bin");
+      }
+      if (IsBin) {
+        llvm::StringRef ParentPath = llvm::sys::path::parent_path(TestPath);
         if (llvm::sys::path::filename(ParentPath) == "VC") {
           Path = ParentPath;
           IsVS2017OrNewer = false;
           return true;
         }
 
       } else {
         // This could be a new (>=VS2017) toolchain. If it is, we should find