[CMake] Added support for disabling building the SourceKit XPC service.

Made `BUILD_SOURCEKIT_XPC_SERVICE` into an option so it can be set externally.
It continues to default to `TRUE` if `HAVE_XPC_H AND SWIFT_BUILD_SOURCEKIT` is
true, and `FALSE` otherwise, but this allows a client to disable this ahead of
time and build a sourcekitd.framework without the XPC service.

This addresses <rdar://problem/85511711>.

Author: cwakamo

tools/CMakeLists.txt

@@ -2,11 +2,15 @@ include(CheckIncludeFiles)
 check_include_files("xpc/xpc.h" HAVE_XPC_H)
 
 if(HAVE_XPC_H AND SWIFT_BUILD_SOURCEKIT)
-  set(BUILD_SOURCEKIT_XPC_SERVICE TRUE)
+  set(BUILD_SOURCEKIT_XPC_SERVICE_default TRUE)
 else()
-  set(BUILD_SOURCEKIT_XPC_SERVICE FALSE)
+  set(BUILD_SOURCEKIT_XPC_SERVICE_default FALSE)
 endif()
 
+option(BUILD_SOURCEKIT_XPC_SERVICE
+  "Whether or not the SourceKit XPC service should be built"
+  ${BUILD_SOURCEKIT_XPC_SERVICE_default})
+
 # Add generated libSyntax headers to global dependencies.
 list(APPEND LLVM_COMMON_DEPENDS swift-syntax-generated-headers)
 if(SWIFT_BUILD_SOURCEKIT)