Ensure that we don't get a very-likely-wrong buffer ID

-1 was once used as a sentinel value for "no buffer"; make sure it
doesn't persist anywhere.

Author: DougGregor

lib/AST/Module.cpp

@@ -3434,6 +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);
   M.getASTContext().addDestructorCleanup(*this);
 
   assert(!IsPrimary || M.isMainModule() &&