[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

clang-tools-extra/CMakeLists.txt

@@ -1,6 +1,13 @@
-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)