[CMake] Workflow improvements to PGO generation

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.

llvm-svn: 278862