Teach the Mach-O-based AST importer to accept bare serialized ASTs.

This necessitated adding a new function to validate a serialized AST, so
that we can get the same information that used to be extracted from the
section header.

For now, we'll continue accepting the wrapped ASTs as well, since we
haven't changed the existing debug info generator.

Swift SVN r12922

lib/Serialization/SerializedModuleLoader.cpp

@@ -253,12 +253,11 @@ SerializedModuleLoader::loadDeclsConformingTo(KnownProtocolKind kind,
   }
 }
 
-bool
-SerializedModuleLoader::isValidSerializedAST(const llvm::MemoryBuffer &input) {
+bool SerializedModuleLoader::isSerializedAST(StringRef data) {
   using serialization::SIGNATURE;
   StringRef signatureStr(reinterpret_cast<const char *>(SIGNATURE),
                          llvm::array_lengthof(SIGNATURE));
-  return input.getBuffer().startswith(signatureStr);
+  return data.startswith(signatureStr);
 }
 
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