[lldb][ExpressionParser][NFC] Clean up expression language picking logic (llvm#156642)

This patch moves the `frame_lang` logic to just the logging (because
that's what it was always used for anyway). The callsites decide whether
to fall back on to the frame language or not when running the
expression.

(cherry picked from commit 25a3045)

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

@@ -579,23 +579,19 @@ static void SetupLangOpts(CompilerInstance &compiler,
   lldb::StackFrameSP frame_sp = exe_scope.CalculateStackFrame();
   lldb::ProcessSP process_sp = exe_scope.CalculateProcess();
 
-  // Defaults to lldb::eLanguageTypeUnknown.
-  lldb::LanguageType frame_lang = expr.Language().AsLanguageType();
-
-  // Make sure the user hasn't provided a preferred execution language with
-  // `expression --language X -- ...`
-  if (frame_sp && frame_lang == lldb::eLanguageTypeUnknown)
-    frame_lang = frame_sp->GetLanguage().AsLanguageType();
+  lldb::LanguageType language = expr.Language().AsLanguageType();
 
-  if (process_sp && frame_lang != lldb::eLanguageTypeUnknown) {
-    LLDB_LOGF(log, "Frame has language of type %s",
-              lldb_private::Language::GetNameForLanguageType(frame_lang));
-  }
+  if (process_sp)
+    LLDB_LOG(
+        log,
+        "Frame has language of type {0}\nPicked {1} for expression evaluation.",
+        lldb_private::Language::GetNameForLanguageType(
+            frame_sp ? frame_sp->GetLanguage().AsLanguageType()
+                     : lldb::eLanguageTypeUnknown),
+        lldb_private::Language::GetNameForLanguageType(language));
 
-  lldb::LanguageType language = expr.Language().AsLanguageType();
   LangOptions &lang_opts = compiler.getLangOpts();
 
-  // FIXME: should this switch on frame_lang?
   switch (language) {
   case lldb::eLanguageTypeC:
   case lldb::eLanguageTypeC89: