Add new LLVM_OPTIMIZED_TABLEGEN build setting which configures, builds and uses a release tablegen build when LLVM is configured with assertions enabled.

Summary: This change leverages the cross-compiling functionality in the build system to build a release tablegen executable for use during the build.

Reviewers: resistor, rnk

Reviewed By: rnk

Subscribers: rnk, joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D7349

llvm-svn: 231842