[SourceKit] Make sourcekitd-test -req=demangle STDIN mode actually work

This didn't work at all because it didn't advance cursor after regex
matching.
Align regex with swift-demangle.

Author: rintaro

tools/SourceKit/tools/sourcekitd-test/sourcekitd-test.cpp

@@ -1663,10 +1663,13 @@ static void prepareDemangleRequest(sourcekitd_object_t Req,
     llvm::StringRef inputContents = input.get()->getBuffer();
 
     // This doesn't handle Unicode symbols, but maybe that's okay.
-    llvm::Regex maybeSymbol("(_T|" MANGLING_PREFIX_STR ")[_a-zA-Z0-9$]+");
+    // Also accept the future mangling prefix.
+    llvm::Regex maybeSymbol("(_T|_?\\$[Ss])[_a-zA-Z0-9$.]+");
     llvm::SmallVector<llvm::StringRef, 1> matches;
     while (maybeSymbol.match(inputContents, &matches)) {
       addName(matches.front());
+      auto offset = matches.front().data() - inputContents.data();
+      inputContents = inputContents.substr(offset + matches.front().size());
     }
 
   } else {

tools/swift-demangle/swift-demangle.cpp

@@ -174,7 +174,6 @@ static void demangle(llvm::raw_ostream &os, llvm::StringRef name,
 static int demangleSTDIN(const swift::Demangle::DemangleOptions &options) {
   // This doesn't handle Unicode symbols, but maybe that's okay.
   // Also accept the future mangling prefix.
-  // TODO: remove the "_S" as soon as MANGLING_PREFIX_STR gets "_S".
   llvm::Regex maybeSymbol("(_T|_?\\$[Ss])[_a-zA-Z0-9$.]+");
 
   swift::Demangle::Context DCtx;