Have llvm-config --cppflags print correct flags when in CMake build directory

Have llvm-config --cppflags print correct flags when in CMake build directory

Previously, if invoked from a CMake build directory, 'llvm-config
--cppflags' and friends would only print a -I flag for the build
directory's header search path, because it would assume that it was
already installed, not recognising its parent directory as being the
build directory.  Teach llvm-config about CMake build directories
so that it prints a -I for both the source and build directory's
search paths.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143171 91177308-0d34-0410-b5e6-96231b3b80d8