[clang] Rename CompilerInvocationBase to RefBase, split out ValueBase

This patch documents the reason `CompilerInvocationBase` exists and renames it to more descriptive `CompilerInvocationRefBase`.

To make the distinction obvious, it also splits out new `CompilerInvocationValueBase` class.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D100455