self-review: correct retrieved start location

- increment the location by one to better point past the initial "
character of a string literal
- add test that demonstrates an HLSLRootSignature parsing error being
propogated up and it correctly continues parsing after the closed paren
enclosing the string literal

Author: inbelic

clang/lib/Parse/ParseDeclCXX.cpp

@@ -5365,8 +5365,10 @@ void Parser::ParseMicrosoftRootSignatureAttributeArgs(ParsedAttributes &Attrs) {
   // Check if we have already found a decl of the same name, if we haven't
   // then parse the root signature string and construct the in-memory elements
   if (!Actions.LookupQualifiedName(R, Actions.CurContext)) {
+    SourceLocation SignatureLoc =
+      StrLiteral.value()->getExprLoc().getLocWithOffset(1); // offset 1 for '"'
     // Invoke the root signature parser to construct the in-memory constructs
-    hlsl::RootSignatureLexer Lexer(Signature, RootSignatureLoc);
+    hlsl::RootSignatureLexer Lexer(Signature, SignatureLoc);
     SmallVector<llvm::hlsl::rootsig::RootElement> Elements;
     hlsl::RootSignatureParser Parser(Elements, Lexer, PP);
     if (Parser.parse()) {

clang/test/SemaHLSL/RootSignature-err.hlsl

@@ -15,3 +15,6 @@ void bad_root_signature_2() {}
 
 [RootSignature(""), RootSignature("")] // expected-warning {{attribute 'RootSignature' is already applied}}
 void bad_root_signature_3() {}
+
+[RootSignature("DescriptorTable(), invalid")] // expected-error {{expected end of stream to denote end of parameters, or, another valid parameter of RootSignature}}
+void bad_root_signature_4() {}