To achieve this the script does a single unified CMake configuration for
llvm/clang/swift and builds only the parser library target.
This mechanism is fundamentally different from build-script, which builds llvm/clang
in a separate build directory than swift.
Even though this bypasses build-script, it does share some underlying helper
utilities from the python infrastructure.
This utility also provides capability to gather profile data and build the parser
library with PGO optimization enabled.
