Get ClangdXPC.framework building (barely) with CMake's Xcode generator

The output directories for CMake's Xcode project generator are
specific to the configuration, and so looking in
CMAKE_LIBRARY_OUTPUT_DIRECTORY isn't going to work. Fortunately, CMake
already provides generator expressions to find the output of a given
target.

I call this "barely" building because the built framework isn't going
to respect the configuration; that is, I can't have both Debug and
RelWithDebInfo variants of ClangdXPC.framework at the same time like I
can with normal library or executable targets. To do that we'd have to
put the framework in a configuration-specific output directory or use
CMake's native support for frameworks instead.

https://reviews.llvm.org/D68846

llvm-svn: 374494