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/doc.h

@@ -125,6 +125,7 @@ enum OperandClass {
     OperandVariableLiteralId,
     OperandLiteralNumber,
     OperandLiteralString,
+    OperandVariableLiteralStrings,
     OperandSource,
     OperandExecutionModel,
     OperandAddressing,