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
11 files changed