build: simplify the blocks handling

`-fblocks` is a core driver option now, so it can be used with both the GCC
style driver as well as the cl style driver.  Simplify the logic for the
handling of this option.

tools/SourceKit/cmake/modules/AddSwiftSourceKit.cmake

@@ -1,4 +1,3 @@
-include(AddSwiftBlocksSupport)
 
 function(add_sourcekit_symbol_exports target_name export_file)
   # Makefile.rules contains special cases for different platforms.
@@ -75,10 +74,9 @@ function(add_sourcekit_default_compiler_flags target)
 
   # Set compilation and link flags.
   set_property(TARGET "${target}" APPEND_STRING PROPERTY
-      COMPILE_FLAGS " ${c_compile_flags}")
+      COMPILE_FLAGS " ${c_compile_flags} -fblocks")
   set_property(TARGET "${target}" APPEND_STRING PROPERTY
       LINK_FLAGS " ${link_flags}")
-  swift_add_blocks_support("${target}")
 endfunction()
 
 # Add a new SourceKit library.