[llvm-dis] Add an option dump-thinlto-index-only in llvm-dis to read ThinLTO minimized code only.

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -3835,6 +3835,11 @@ Error BitcodeReader::parseModule(uint64_t ResumeBit,
       if (Error Err = parseComdatRecord(Record))
         return Err;
       break;
+    // FIXME: BitcodeReader should handle {GLOBALVAR, FUNCTION, ALIAS, IFUNC}
+    // written by ThinLinkBitcodeWriter. See
+    // `ThinLinkBitcodeWriter::writeSimplifiedModuleInfo` for the format of each
+    // record
+    // (https://github.com/llvm/llvm-project/blob/b6a93967d9c11e79802b5e75cec1584d6c8aa472/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4714)
     case bitc::MODULE_CODE_GLOBALVAR:
       if (Error Err = parseGlobalVarRecord(Record))
         return Err;