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

clang currently warns when passing flags for the assembler (e.g.
-Wa,-mbig-obj) to an invocation that doesn't run the assembler (e.g.
-E).

At first sight, that makes sense -- the flag really is unused. But many
other flags don't have an effect if no assembler runs (e.g.
-fno-integrated-as, -ffunction-sections, and many others), and those
currently don't warn. So this seems more like a side effect of how
CollectArgsForIntegratedAssembler() is implemented than like an
intentional feature.

Since it's a bit inconvenient when debugging builds and adding -E,
always call CollectArgsForIntegratedAssembler() to make sure assembler
args always get claimed. Currently, this affects only these flags:
-mincremental-linker-compatible, -mimplicit-it= (on ARM), -Wa, -Xassembler

It does have the side effect that assembler options now need to be valid
even if -E is passed. Previously, `-Wa,-mbig-obj` would error for
non-coff output only if the assembler ran, now it always errors. This
too makes assembler flags more consistent with all the other flags and
seems like a progression.

Fixes PR42066.

Differential Revision: https://reviews.llvm.org/D64527

llvm-svn: 365703