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

llvm-svn: 206984
8 files changed