'clang -cc1' and 'clang -cc1as' will mutate the triple to account for
-target-abi if such a conversion is defined by Triple::getABIVariant().
Otherwise, the ABI will continue to be passed via MCTargetOptions::ABIName.
Additionally, 'clang -cc1as' now applies the effect of -target-abi. Previously
it was ignored.
Patch by: Daniel Sanders