Ungroup "no unsafe operations occur within 'unsafe'" warnings

These should not be escalated to errors when other strict memory safety
warnings are, because they aren't safety issues. They could go into a
separate group along with the corresponding try and await diagnostics.

(cherry picked from commit 02e1d11)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -8364,9 +8364,9 @@ GROUPED_WARNING(unsafe_without_unsafe,StrictMemorySafety,none,
       "expression uses unsafe constructs but is not marked with 'unsafe'", ())
 GROUPED_WARNING(for_unsafe_without_unsafe,StrictMemorySafety,none,
       "for-in loop uses unsafe constructs but is not marked with 'unsafe'", ())
-GROUPED_WARNING(no_unsafe_in_unsafe,StrictMemorySafety,none,
+WARNING(no_unsafe_in_unsafe,none,
         "no unsafe operations occur within 'unsafe' expression", ())
-GROUPED_WARNING(no_unsafe_in_unsafe_for,StrictMemorySafety,none,
+WARNING(no_unsafe_in_unsafe_for,none,
         "no unsafe operations occur within 'unsafe' for-in loop", ())
 NOTE(make_subclass_unsafe,none,
      "make class %0 '@unsafe' to allow unsafe overrides of safe superclass "