[clang] Don't pass multiple backend options if mixing -mimplicit-it and -Wa,-mimplicit-it

If multiple instances of the -arm-implicit-it option is passed to
the backend, it errors out.

Also fix cases where there are multiple -Wa,-mimplicit-it; the existing
tests indicate that the last one specified takes effect, while in
practice it passed double options, which didn't work as intended.

Differential Revision: https://reviews.llvm.org/D102812