[BOLT] Prepare BOLT for unit-testing

This patch adds unit testing support for BOLT. In order to do this we will need at least do this changes on the code level:
* Make createMCPlusBuilder accessible externally
* Remove positional InputFilename argument to bolt utlity sources
And prepare the cmake and lit for the new tests.

Vladislav Khmelevsky,
Advanced Software Technology Lab, Huawei

Reviewed By: maksfb, Amir

Differential Revision: https://reviews.llvm.org/D118271