Code cleanup in preparation for adding LTO for wasm. NFC.

- Move some common code into Common/rrorHandler.cpp and
  Common/Strings.h.
- Don't use `fatal` when incompatible bitcode files are
  encountered.
- Rename NameRef variable to just Name

See D47162

Differential Revision: https://reviews.llvm.org/D47206

llvm-svn: 333021