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/SPVRemapper.cpp

@@ -625,6 +625,9 @@ namespace spv {
                 break;
             }
 
+            case spv::OperandVariableLiteralStrings:
+                return nextInst;
+
             // Execution mode might have extra literal operands.  Skip them.
             case spv::OperandExecutionMode:
                 return nextInst;