[lldb][NFC] Extract searching for function SymbolContexts out of...

[lldb][NFC] Extract searching for function SymbolContexts out of ClangExpressionDeclMap::LookupFunction

This code was just creating a new SymbolContextList with any found functions
in the front and orders them by how close they are to the current frame.
This refactors this code into its own function to make this more obvious.

Doesn't do any other changes to the code, so this is NFC.