[asan][Darwin] Unprotected use of Apple blocks breaks build with GCC

[fxcoudert]

The commit at 020cdaf broke build of asan on macOS with GCC. GCC does not support the Apple blocks extension (yet). Uses of blocks in other parts of the sanitisers are protected by MISSING_BLOCKS_SUPPORT. But the type definition is not.

Could the same thing be done here? This works for me (tested in a build of asan inside GCC):

diff --git a/libsanitizer/asan/asan_mac.cpp b/libsanitizer/asan/asan_mac.cpp
index 5d5146e0cde..6252fa20d5e 100644
--- a/libsanitizer/asan/asan_mac.cpp
+++ b/libsanitizer/asan/asan_mac.cpp
@@ -139,9 +139,11 @@ typedef void (*dispatch_mach_handler_function_t)(void *context,
                                                  dispatch_mach_reason reason,
                                                  dispatch_mach_msg_t message,
                                                  mach_error_t error);
+#if !defined(MISSING_BLOCKS_SUPPORT)
 typedef void (^dispatch_mach_handler_t)(dispatch_mach_reason reason,
                                         dispatch_mach_msg_t message,
                                         mach_error_t error);
+#endif
 
 // A wrapper for the ObjC blocks used to support libdispatch.
 typedef struct {

[thesamesam]

Could you submit that as a PR? Thanks.

[thesamesam]

cc @thetruestblue