[patch] Fix pr16247

[Rafael Espíndola]

The attached patch implements the restriction:

An entity with C language linkage shall not be declared with
the same name as an entity in global scope, unless both declarations
denote the same entity.

It does it by just avoiding overloading in that case. The existing
error checking for redeclarations then reject the different
signatures.

One part of the patch I am not sure about is the changes to the ObjC
rewriter. It was depending on overloading the extern "C" function
objc_enumerationMutation.

Cheers,
Rafael
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