swift234

[cmake] Expand $(CONFIGURATION)$(EFFECTIVE_PLATFORM_NAME) in LLVM CMake paths.

In order to support swapping build types without reconfiguring, Swift's CMake
generated Xcode project has one directory for each cmake build type. This
behavior is implemented by placing in variables for Xcode to expand at build
time. This causes issues with our build, namely that our cmake scripts like to
"poke" around the LLVM build directory at configure time. Since the xcode
variables from the commit title are not expanded, failures occur since the
unexpanded paths do not exist.

This patch fixes this problem by expanding out those variables at configure time
from specific LLVM paths. This is essentially tieing LLVM to the specific
build-script build mode used. This is reasonable since we do not support
building in Xcode for configurations that were not selected at build-script
time.

rdar://27062396