SPIRV: Add disassembly support for multiple literal strings (#2397)

According to the extension SPV_GOOGLE_decorate_string,
OpDecorateString (or OpMemberDecorateString) ought to be capable of
supporting multiple literal strings. Each literal strings are padded
with null terminator to make word alignment. The layout is:

  Inst | Target | Decoration | Literal String, Literal String, ...

SPIRV/disassemble.cpp

@@ -519,6 +519,10 @@ void SpirvStream::disassembleInstruction(Id resultId, Id /*typeId*/, Op opCode,
         case OperandLiteralString:
             numOperands -= disassembleString();
             break;
+        case OperandVariableLiteralStrings:
+            while (numOperands > 0)
+                numOperands -= disassembleString();
+            return;
         case OperandMemoryAccess:
             outputMask(OperandMemoryAccess, stream[word++]);
             --numOperands;