[lldb-vscode] Use expression command for completion

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.

Differential Revision: https://reviews.llvm.org/D145437

(cherry picked from commit e221dab)

Author: kastiglione

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

@@ -1062,8 +1062,9 @@ void request_completions(const llvm::json::Object &request) {
     text = text.substr(1);
     actual_column--;
   } else {
-    text = "p " + text;
-    actual_column += 2;
+    char command[] = "expression -- ";
+    text = command + text;
+    actual_column += strlen(command);
   }
   lldb::SBStringList matches;
   lldb::SBStringList descriptions;