Roll LLVM to r214593

Incorporates a couple of sanitizer bug fixes and a fix for an assertion
failure which broke the libgo test suite.

Also build the FileCheck tool required by llgo's test suite when using Ninja.
Expose the LLVM version as a string constant

We retrieve the version from llvm-config and store it in the repository
to support the case where byollvm is defined.
Merge pull request #35 from pcc/roll

Roll LLVM to r214593 and expose the LLVM version as a string constant