[lldb] NFC: refactor CompileUnit::ResolveSymbolContext

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

a) many nested blocks,
b) one return statement at the end with some logic involved,
c) a duplicated while-loop with just small differences in it.

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.
I also replaced the two while-loops with just one.

This is a non-functional change (NFC).

Reviewers: jdoerfert, teemperor

Reviewed By: teemperor

Subscribers: labath, teemperor, lldb-commits

Tags: #lldb

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