[Driver] Add support for -fembed-bitcode for assembly file

Summary:
Handle -fembed-bitcode for assembly inputs. When the input file is
assembly, write a marker as "__LLVM,__asm" section.

Fix llvm.org/pr39659

Reviewers: compnerd, dexonsmith

Reviewed By: compnerd

Subscribers: rjmccall, dblaikie, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D55525

llvm-svn: 348943