Add llvm-exegesis tool.

Summary:
[llvm-exegesis][RFC] Automatic Measurement of Instruction Latency/Uops

This is the code corresponding to the RFC "llvm-exegesis Automatic Measurement of Instruction Latency/Uops".

The RFC is available on the LLVM mailing lists as well as the following document
for easier reading:
https://docs.google.com/document/d/1QidaJMJUyQdRrFKD66vE1_N55whe0coQ3h1GpFzz27M/edit?usp=sharing

Subscribers: mgorny, gchatelet, orwant, llvm-commits

Differential Revision: https://reviews.llvm.org/D44519

llvm-svn: 329156