Silence warning about mismatched signs

Author: DougGregor

lib/AST/Module.cpp

@@ -3434,7 +3434,7 @@ SourceFile::SourceFile(ModuleDecl &M, SourceFileKind K,
                        ParsingOptions parsingOpts, bool isPrimary)
     : FileUnit(FileUnitKind::Source, M), BufferID(bufferID),
       ParsingOpts(parsingOpts), IsPrimary(isPrimary), Kind(K) {
-  assert(BufferID != ~0);
+  assert(BufferID != (unsigned)~0);
   M.getASTContext().addDestructorCleanup(*this);
 
   assert(!IsPrimary || M.isMainModule() &&