[lldb] NFC: refactor CompileUnit::ResolveSymbolContext

Summary:
I found the above named method hard to read because it had

a) many nested blocks and
b) one return statement at the end with some logic involved.

I decided to refactor this function by employing an early exit strategy.
In order to capture the logic in the return statement and to not have it
repeated more than once I chose to implement a very small lamda function
that captures all the variables it needs.

This is a non-functional change (NFC).

Reviewers: jdoerfert

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70774