[clangd][xpc][cmake] Respect explicit value of CLANGD_BUILD_XPC

We shouldn't prevent user from disabling XPC framework build on Darwin.
However, by keeping it on by default our CI systems also test
it by default on macOS.

Based on user request:
http://lists.llvm.org/pipermail/cfe-dev/2019-March/061778.html

Differential Revision: https://reviews.llvm.org/D59808

llvm-svn: 356974