Merge backported retpoline patches for clang 5.0.1

[dim]

Summary

This merges the following backported retpoline patches into stable/11's
version of clang 5.0.1:

https://reviews.llvm.org/rL324007
https://reviews.llvm.org/rL324009
https://reviews.llvm.org/rL324012
https://reviews.llvm.org/rL324025
https://reviews.llvm.org/rL324026
https://reviews.llvm.org/rL325088
https://reviews.llvm.org/rL325089
https://reviews.llvm.org/rL325090
https://reviews.llvm.org/rL325091

Test Plan

Going to build a universe with this before committing, and check whether
the command line options work as advertised.

[emaste]

IndirectBrExpandPass.cpp and X86RetpolineThunks.cpp have unintended SVN properties in the diff it seems

[dim]

In D14720#309378, @emaste wrote:

IndirectBrExpandPass.cpp and X86RetpolineThunks.cpp have unintended SVN properties in the diff it seems

Yeah, these get 'helpfully' added by Subversion automatically. I'll get rid of them.

[dim]

Remove auto-added Subversion properties.

[dim]

This has been committed now.