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. An
additional function clang_isBitFieldDecl() is added to disambiguate
between error code meanings.