[libclang] No longer attempt to get a dependent bit-width

Handle template parameter-dependent bit field widths in libclang

In a class template, a bit field's width may depend on a template
parameter. In this case the width expression cannot be evaluated.

Previously clang_getFieldDeclBitWidth() would assert, or cause memory
unsafety and return an invalid result if assertions are disabled.

This adds a check for this case which returns an error code.

This work was largely taken from an earlier patch which was reverted
due to an accidental API duplication
(https://reviews.llvm.org/D130303).

Fixes: https://github.com/llvm/llvm-project/issues/56644


Co-authored-by: Aaron Ballman <aaron@aaronballman.com>
Differential Revision: https://reviews.llvm.org/D146039