[clang][ARM] When handling multiple -mimplicit-it mark all as used

Since 4468e5b8 clang will prefer
the last one it finds of "-mimplicit-it" or "-Wa,-mimplicit-it".

Due to a mistake in that patch the compiler argument "-mimplicit-it"
was never marked as used, even if it was the last one and was passed
to llvm.

Move the Claim call back to the start of the loop and update
the testing to check we don't get any unused argument warnings.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D103086