[HLSL][RootSignature] Incorrectly allows specifying parameters without a comma

[inbelic]

This issue tracks fixing a bug that allows parameters to be specified without an intermediate comma.

For instance:

RootFlags(0) CBV(b0)

Should be invalid because there is no comma between the two parameters.

This is because of the code pattern as follows:

llvm-project/clang/lib/Parse/ParseHLSLRootSignature.cpp

Lines 30 to 42 in 3c76a05

	if (tryConsumeExpectedToken(TokenKind::kw_RootFlags)) {
	auto Flags = parseRootFlags();
	if (!Flags.has_value())
	return true;
	Elements.push_back(*Flags);
	}
	if (tryConsumeExpectedToken(TokenKind::kw_RootConstants)) {
	auto Constants = parseRootConstants();
	if (!Constants.has_value())
	return true;
	Elements.push_back(*Constants);
	}

It should be an else-if chain so that if we successfully parse the first element type, it will not attempt to parse any other element types right after.

Reproduction here: https://godbolt.org/z/arcExM9rE

AC:

• All occurrences of the if chains should be replaced with an else-if chain
• Add unit tests for this scenario