sys::findProgramByName(): [Win32] Tweak to pass lowercase .exe to SearchPath() to appease clang Driver's tests.

It seems SearchPath() doesn't show actual extension on the filesystem.

FIXME: Shall we use FindFirstFile() here?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221246 91177308-0d34-0410-b5e6-96231b3b80d8