[clang] Implement CompilerInvocation copy assignment

This patch implements the copy assignment for `CompilerInvocation`.

Eventually, the deep-copy operation will be moved into a `clone()` method (D100460), but until then, this is necessary for basic ergonomics.

Depends on D100455.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D100473