Add new 'let' command to bind arbitrary values into constants.

Summary:
Add new 'let' command to bind arbitrary values into constants.
These constants can then be used in the matcher expressions.

Reviewers: pcc

CC: cfe-commits

Differential Revision: http://reviews.llvm.org/D3383

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@206984 91177308-0d34-0410-b5e6-96231b3b80d8