[build] SourceKit no longer cross-compiles with the Swift 5.3 snapshots

There was a mixup in swiftlang#29451 and a CMake flag was dropped, this puts it back.

Resolves SR-13315

Author: finagolfin

tools/SourceKit/cmake/modules/AddSwiftSourceKit.cmake

@@ -78,7 +78,7 @@ function(add_sourcekit_default_compiler_flags target)
       ${${SWIFT_HOST_VARIANT_ARCH}_INCLUDE})
   endif()
   target_compile_options(${target} PRIVATE
-    -fblocks)
+    ${c_compile_flags} -fblocks)
   target_link_options(${target} PRIVATE
     ${link_flags})
   target_link_directories(${target} PRIVATE