[LTO] Add options to llvm-lto to select output format and dump merged module

This introduces two new options:
- "llvm-lto -save-merged-module -o outfile" dumps the LTO Module to
  outfile.merged.bc prior to CodeGen and after LTO optimizations have been run.
- "llvm-lto -filetype=asm -o outfile" makes llvm-lto emit assembly instead of
  object code in outfile.

Both are intended for use in lit tests.

llvm-svn: 253624