Remove lookupSymbol() and have all callers use SymbolInfo::lookup() instead

[grynspan]

This PR removes the free function lookupSymbol() and modifies its callers to use SymbolInfo::lookup() instead.

In a previous PR, I refactored lookupSymbol() to call through to SymbolInfo::lookup() which itself returned an llvm::Optional<SymbolInfo>, which lookupSymbol() unpacks and copies into an out-param. As discussed in the review for that PR, we might as well remove the stub function and update callers to just use the underlying function.

[grynspan]

@swift-ci please test

[al45tair]

LGTM. I do have reservations about auto (I don't like that it hides the type at the call site, which makes the code much harder to read because now you have to find the declaration to understand what e.g. getFilename() might refer to), but I think I'd also accept that using it is idiomatic C++ these days and I'm doubtless guilty of using it myself in places where later on I might wish I hadn't.

[grynspan]

LGTM. I do have reservations about auto (I don't like that it hides the type at the call site, which makes the code much harder to read because now you have to find the declaration to understand what e.g. getFilename() might refer to), but I think I'd also accept that using it is idiomatic C++ these days and I'm doubtless guilty of using it myself in places where later on I might wish I hadn't.

I've got another PR coming up to enable getFilename() on Windows, and it requires some refactoring anyway to avoid use-after-free scenarios, so I'll revisit my use of auto in that one.