clang can't interpret path with drive letter on win32.

D:\>clang -v
clang version 2.9 (tags/RELEASE_29/final)
Target: i386-pc-mingw32
Thread model: posix

D:\>clang d:\a.c
error: error reading 'd:\a.c'
1 error generated.

D:\>clang a.c

D:\>

This should be fixed with r132539 (trunk)