[lldb-vscode] Use `expression` command for completion

[kastiglione]

Summary

Change lldb-vscode to use the expression command for generating completions, instead
of the p alias.

Aliases are user overrideable, and even deletable, but the expression command is
unchangeable.

See D141539 where a similar replacement was done to tests.

[aprantl]

That seems to be clearly better, since p could be aliased to anything.

[aprantl]

We might consider using dwim-print here in the future?

[kastiglione]

Fix adjacent code and make the tests pass

lldb/tools/lldb-vscode/lldb-vscode.cpp

void request_completions(const llvm::json::Object &request) {
   // TODO: support frameID. Currently
   // g_vsc.debugger.GetCommandInterpreter().HandleCompletionWithDescriptions
   // is frame-unaware.
 
   if (!text.empty() && text[0] == '`') {
     text = text.substr(1);
     actual_column--;
   } else {
-    text = "p " + text;
+    text = "expression -- " + text;
     actual_column += 2;
   }
   lldb::SBStringList matches;
   lldb::SBStringList descriptions;
   g_vsc.debugger.GetCommandInterpreter().HandleCompletionWithDescriptions(
       text.c_str(), actual_column, 0, -1, matches, descriptions);
   size_t count = std::min((uint32_t)100, matches.GetSize());
   targets.reserve(count);