Add documentation for sample profiling support.

Summary:
This documents the usage of sample profilers with Clang and the
profile format expected by LLVM's optimizers. It also documents the
profile conversion tool used by Linux Perf.

Reviewers: doug.gregor

CC: cfe-commits

Differential Revision: http://reviews.llvm.org/D3402

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206994 91177308-0d34-0410-b5e6-96231b3b80d8
Review feedback

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206995 91177308-0d34-0410-b5e6-96231b3b80d8
Review feedback.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206996 91177308-0d34-0410-b5e6-96231b3b80d8
Review feedback.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206997 91177308-0d34-0410-b5e6-96231b3b80d8