[CMake] Support a simple case for bootstrap builds to generate PGO data

[CMake] Support a simple case for bootstrap builds to generate PGO data

Summary:
This patch adds support for the clang multi-stage bootstrapping to support PGO profdata generation, and can build a 2 or 3 stage compiler.

With this patch applied you can configure your build directory with the following invocation of CMake:

cmake -G <generator> -C <path_to_clang>/cmake/caches/PGO.cmake <source dir>

After configuration the following additional targets will be generated:

stage2-instrumented:
Builds a stage1 x86 compiler, runtime, and required tools (llvm-config, llvm-profdata) then uses that compiler to build an instrumented stage2 compiler.

stage2-instrumented-generate-profdata:
Depends on "stage2-instrumented" and will use the instrumented compiler to generate profdata based on the training files in <clang>/utils/perf-training

stage2:
Depends on "stage2-instrumented-generate-profdata" and will use the stage1 compiler with the stage2 profdata to build a PGO-optimized compiler.

stage2-check-llvm:
Depends on stage2 and runs check-llvm using the stage3 compiler.

stage2-check-clang:
Depends on stage2 and runs check-clang using the stage3 compiler.

stage2-check-all:
Depends on stage2 and runs check-all using the stage3 compiler.

stage2-test-suite:
Depends on stage2 and runs the test-suite using the stage3 compiler (requires in-tree test-suite).

Reviewers: bogner, silvas, chandlerc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15584

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@256069 91177308-0d34-0410-b5e6-96231b3b80d8