[Sema] Enable suppression of inferred conformances.

Add the machinery to support suppression of inference of conformance to
protocols that would otherwise be derived automatically.

This commit does not enable any conformances to be suppressed.

lib/Serialization/Serialization.cpp

@@ -3921,6 +3921,8 @@ public:
       typeRef = (typeRef << 1) | (inherited.isUnchecked() ? 0x01 : 0x00);
       // Encode "preconcurrency" in the low bit.
       typeRef = (typeRef << 1) | (inherited.isPreconcurrency() ? 0x01 : 0x00);
+      // Encode "suppressed" in the next bit.
+      typeRef = (typeRef << 1) | (inherited.isSuppressed() ? 0x01 : 0x00);
 
       result.push_back(typeRef);
     }