Fix a 32-bit overflow issue when reading LTO-generated bitcode files whose...

Fix a 32-bit overflow issue when reading LTO-generated bitcode files whose strtab are of size > 2^29

This happens when using -flto and -Wl,--plugin-opt=emit-llvm to create a linked LTO bitcode file, and the bitcode file has a strtab with size > 2^29.

All the issues relate to a pattern like this
  size_t x64 = y64 + z32 * C
  When z32 is >= (2^32)/C, z32 * C overflows.

Reviewed-by: MaskRay

Differential Revision: https://reviews.llvm.org/D86500