Fix program name in "clang -help"

With out this patch,
"clang -help" prints "USAGE: clang-3 [options] <inputs>".
It should either print
USAGE: clang [options] <inputs>
or

USAGE: clang-3.7 [options] <inputs>
With this patch, on Linux, it prints
USAGE: clang-3.7 [options] <inputs>

On Windows, it prints
USAGE: clang.exe [options] <inputs>

llvm-svn: 231124