[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.