[cleanup] Factor out adjusting "cl.exe" to "clang-cl.exe". NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215754 91177308-0d34-0410-b5e6-96231b3b80d8
[cleanup] Factor out initializing the DianosticOptions. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215755 91177308-0d34-0410-b5e6-96231b3b80d8
[cleanup] Factor out setting the driver's install dir. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215756 91177308-0d34-0410-b5e6-96231b3b80d8