[PATCH][ObjC] Cleanup ObjCInterfaceDecl lookup for ObjC literals

[AlexDenisov]

ObjC literals (NSDictionary, NSArray, Boxed Expressions) use the same
code to instantiate ObjCInterfaceDecl of corresponding classes.

The patch extracts duplicated code into one method and unifies
validation of the ObjCInterfaceDecl’s.

Also, diagnostic for NSDictionary/NSArray was changed a bit.
Before:

@class NSDictionary;
// …
id dictionary = @{}; // expected-error {{declaration of 'dictionaryWithObjects:forKeys:count:' is missing in NSDictionary class}}

After:

@class NSDictionary; // expected-note {{forward declaration of class here}}
// …
id dictionary = @{}; // expected-error {{NSDictionary must be available to use Objective-C dictionary literals}}

Implementation ‘before’ is correct, but it’s not that precise and might be confusing.
--
AlexDenisov
Software Engineer,
http://lowlevelbits.org

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