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

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