driver: Don't warn about assembler flags being unused when not assembling; different approach

This morally relands r365703 (and r365714), originally reviewed at
https://reviews.llvm.org/D64527, but with a different implementation.

Relanding the same approach with a fix for the revert reason got a bit
involved (see https://reviews.llvm.org/D65108) so use a simpler approach
with a more localized implementation (that in return duplicates code
a bit more).

This approach also doesn't validate flags for the integrated assembler
if the assembler step doesn't run.

Fixes PR42066.

Differential Revision: https://reviews.llvm.org/D65233

llvm-svn: 367165