[lldb][NFC] Make all CompilerDeclContext parameters references instead of pointers

Summary:
All of our lookup APIs either use `CompilerDeclContext &` or `CompilerDeclContext *` semi-randomly it seems.
This leads to us constantly converting between those two types (and doing nullptr checks when going from
pointer to reference). It also leads to the confusing situation where we have two possible ways to express
that we don't have a CompilerDeclContex: either a nullptr or an invalid CompilerDeclContext (aka a default
constructed CompilerDeclContext).

This moves all APIs to use references and gets rid of all the nullptr checks and conversions.

Reviewers: labath, mib, shafik

Reviewed By: labath, shafik

Subscribers: shafik, arphaman, abidh, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74607