Driver: refactor a local variable

Pull out a getToolChain() into a local variable to share the call across to all
the uses.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224653 91177308-0d34-0410-b5e6-96231b3b80d8
Driver: hoist an assertion

Remove an unnecessary conditional, hoisting the assertion.  Minor style
tweaks/reflowing.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224654 91177308-0d34-0410-b5e6-96231b3b80d8