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

[jkorous]

Summary

We shouldn't prevent user from disabling XPC framework build on Darwin. However, I prefer to keep it on by default so 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

[arphaman]

LG, Thanks

clang-tools-extra/trunk/CMakeLists.txt

-option(CLANGD_BUILD_XPC "Build XPC Support For Clangd." OFF)
-if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
-  set(CLANGD_BUILD_XPC ON CACHE BOOL "" FORCE)
+if (NOT DEFINED CLANGD_BUILD_XPC)
+  if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+    set(CLANGD_BUILD_XPC_DEFAULT ON)
+  else ()
+    set(CLANGD_BUILD_XPC_DEFAULT OFF)
+  endif ()
+
+  set(CLANGD_BUILD_XPC ${CLANGD_BUILD_XPC_DEFAULT} CACHE BOOL "Build XPC Support For Clangd." FORCE)
+
+  unset(CLANGD_BUILD_XPC_DEFAULT)
 endif ()
 
 add_subdirectory(clang-apply-replacements)
 add_subdirectory(clang-reorder-fields)
 add_subdirectory(modularize)
 add_subdirectory(clang-tidy)
 add_subdirectory(clang-tidy-vs)