Use llvm::sys::fs::getMainExecutable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184915 91177308-0d34-0410-b5e6-96231b3b80d8
Remove PathV1.h from CompilerInvocation.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184918 91177308-0d34-0410-b5e6-96231b3b80d8