check-clang-tools: Actually build and run XPC test

The CMake variable controlling if XPC code is built is called
CLANGD_BUILD_XPC but three places unintentionally checked the
non-existent variable CLANGD_BUILD_XPC_SUPPORT instead, which (due to
being nonexistent, and due to cmake) always silently evaluated to false.

Luckily the test still seems to pass, despite never running after being
added almost 3 months ago in r351280.

Differential Revision: https://reviews.llvm.org/D60120

llvm-svn: 357719