Eliminate unnecessary file access checks in Clang driver on Windows

[amccarth]

Summary

This fixes the problem where the driver will look for cl.exe.exe while walking the PATH.

I looked into changing the Windows implementation of llvm::sys::fs::can_execute(), but there wasn't a satisfying way to preserve the extra checks for the callers that depend on it. I also looked into having it actually check that the file is an executable, but this seemed to add little value at the cost of more file i/o.

[majnemer]

LGTM

cfe/trunk/lib/Driver/MSVCToolChain.cpp

if (OptPath.hasValue()) {
       llvm::SplitString(OptPath.getValue(), PathSegments, EnvPathSeparatorStr);
 
       for (StringRef PathSegment : PathSegments) {
         if (PathSegment.empty())
           continue;
 
         SmallString<128> FilePath(PathSegment);
         llvm::sys::path::append(FilePath, "cl.exe");
-        if (llvm::sys::fs::can_execute(FilePath.c_str()) &&
+        // Checking if cl.exe exists is a small optimization over calling
+        // can_execute, which really only checks for existence but will also do
+        // extra checks for cl.exe.exe.  These add up when walking a long path.
+        if (llvm::sys::fs::exists(FilePath.c_str()) &&
             !llvm::sys::fs::equivalent(FilePath.c_str(), clangProgramPath)) {
           // If we found it on the PATH, use it exactly as is with no
           // modifications.
           path = PathSegment;
           return true;
         }
       }
     }