[lldb] Re-use clang's keyword enable/disable mechanism in CPlusPlusNameParser.cpp

The problem is that CPlusPlusNameParser doesn't respect the language dialect
for `target variable Class::constant`. If `constant` is a keyword in some
(downstream in this case) dialects then it lexes `constant` as `kw_constant`
instead of `kw_raw_identifier` even if that dialect is not active. As a
result:
* `target variable constant` works
* `target variable Class::constant` fails to look up constant because it sees
  `kw_raw_identifier, kw_coloncolon, kw_constant` instead of
  `kw_raw_identifier, kw_coloncolon, kw_raw_identifier`
* `expr -l c++ -- Class::constant` works because clang is parsing it

Note: There's seemingly a separate bug where GetLangOptions() does not account
for the process/frame language and just uses a pre-defined set of language
options. I have not attempted to fix that since, for now, at least
hardcoding my dialect to be disabled by that function does the
right thing for existing tests.

Also fixed a trivial return-after-else that clangd pointed out in the same
area

Author: dsandersllvm

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusNameParser.cpp

@@ -719,9 +719,8 @@ CPlusPlusNameParser::ParseFullNameImpl() {
     }
     start_position.Remove();
     return result;
-  } else {
-    return std::nullopt;
   }
+  return std::nullopt;
 }
 
 llvm::StringRef CPlusPlusNameParser::GetTextForRange(const Range &range) {
@@ -755,12 +754,19 @@ static const clang::LangOptions &GetLangOptions() {
   return g_options;
 }
 
-static const llvm::StringMap<tok::TokenKind> &GetKeywordsMap() {
-  static llvm::StringMap<tok::TokenKind> g_map{
-#define KEYWORD(Name, Flags) {llvm::StringRef(#Name), tok::kw_##Name},
+static const llvm::StringMap<tok::TokenKind> GetKeywordsMap() {
+  using namespace clang;
+  llvm::StringMap<tok::TokenKind> g_map;
+
+  auto LangOpts = GetLangOptions();
+
+  KeywordStatus AddResult;
+#define KEYWORD(NAME, FLAGS)                                                   \
+  AddResult = getKeywordStatus(LangOpts, FLAGS);                               \
+  if (AddResult != KS_Disabled)                                                \
+    g_map[llvm::StringRef(#NAME)] = tok::kw_##NAME;
 #include "clang/Basic/TokenKinds.def"
 #undef KEYWORD
-  };
   return g_map;
 }
 
@@ -769,7 +775,7 @@ void CPlusPlusNameParser::ExtractTokens() {
     return;
   clang::Lexer lexer(clang::SourceLocation(), GetLangOptions(), m_text.data(),
                      m_text.data(), m_text.data() + m_text.size());
-  const auto &kw_map = GetKeywordsMap();
+  const auto kw_map = GetKeywordsMap();
   clang::Token token;
   for (lexer.LexFromRawLexer(token); !token.is(clang::tok::eof);
        lexer.LexFromRawLexer(token)) {