[LLVM-C] Miscellaneous Cleanups in DIBuilder Bindings

Summary:
* rL328953 does not include bindings for LLVMDIBuilderCreateClassType and LLVMDIBuilderCreateBitFieldMemberType despite declaring their prototypes.  Provide these bindings now.
* Switch to more precise types with specific numeric limits matching the DIBuilder's C++ API.

Reviewers: harlanhaskins, whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46168

llvm-svn: 331114