Bitcode: Introduce BitcodeWriter interface. This interface allows clients to write multiple modules to a single bitcode file. Also introduce the llvm-cat utility which can be used to create a bitcode file containing multiple modules. Differential Revision: https://reviews.llvm.org/D26179 llvm-svn: 288195 GitOrigin-RevId: 5a0a2e648c267d99111b21482ca709f580e9ccc2