swift234

[Bitcode Driver] make sure -embed-bitcode works with -emit-module.

With both emit-module and embed-bitcode, MergeModule will get the swiftmodule
inputs from CompileJobAction instead of BackendJobAction.

This commit should only affect Bitcode mode since it only touches how we handle
BackendJobAction which is used for Bitcode mode.

rdar://20678489


Swift SVN r27878