clang/Driver: Use struct type for BoundArch instead of StringRef (#204748)
Change BoundArch arguments in the clang driver from StringRef (or
sometimes const char*) to a dedicated struct type that contains both
the architecture string and a parsed OffloadArch enum field. In the
future it may be useful to contain other feature bits here.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>