clang/lib/Driver/Driver.cpp: Split COMPILER_PATH according to...

clang/lib/Driver/Driver.cpp: Split COMPILER_PATH according to llvm::sys::PathSeparator, that is ';' in Win32 hosts.

Thanks to Bogon Kim!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169964 91177308-0d34-0410-b5e6-96231b3b80d8