[NFC][lldb] Make 2 static funcs in SwiftExpressionParser member funcs

In preparation of refactoring SetupASTContext to use a call_once for
initialization, make SetupASTContext and ParseAndImport member
functions. This is needed so SwiftExpressionParser can acquire a
ThreadSafeASTContext multiple times, so it doesn't need to hold on to
that lock for the entirety of expression parsing.

Author: augusto2112