[clang] Make libBasic not depend on MC

Reduces numbers of files built for clang-format from 575 to 449.

Requires two small changes:

1. Don't use llvm::ExceptionHandling in LangOptions. This isn't
   even quite the right type since we don't use all of its values.
   Tweaks the changes made in:
   - https://reviews.llvm.org/D93215
   - https://reviews.llvm.org/D93216

2. Move section name validation code added (long ago) in commit 30ba6743 out
   of libBasic into Sema and base the check on the triple. This is a bit less
   OOP-y, but completely in line with what we do in many other places in Sema.

No behavior change.

Differential Revision: https://reviews.llvm.org/D101463