[LTO] Add option to emit assembly from LTOCodeGenerator

Description

[LTO] Add option to emit assembly from LTOCodeGenerator

This adds a new API, LTOCodeGenerator::setFileType, to choose the output file
format for LTO CodeGen. A corresponding change to use this new API from
llvm-lto and a test case is coming in a separate commit.

Differential Revision: http://reviews.llvm.org/D14554