This patch adds a few new convenience options used by the PGO CMake cache to setup options on bootstrap stages. The new options are:

PGO_INSTRUMENT_LTO - Builds the instrumented and final builds with LTO
PGO_BUILD_CONFIGURATION - Accepts a CMake cache script that can be used for complex configuration of the stage2-instrumented and stage2 builds.

The patch also includes a fix for bootstrap dependencies so that the instrumented LTO tools don't get used when building the final stage, and it adds distribution targets to the passthrough.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@278862 91177308-0d34-0410-b5e6-96231b3b80d8