almost all calls to LookupSingleName are wrong

[ec04fc15-fa35-46f2-80e1-5d271f2ef708]

Bugzilla Link	25551
Version	trunk
OS	Linux
CC	@riccibruno

Extended Description

LookupSingleName does something very odd: if lookup finds an overload set of functions, it acts exactly like lookup found nothing. This results in almost every single caller of this function doing the wrong thing in that case (ranging from misdiagnosing problems to accepting invalid code). Example:

int m(); enum { m }; // error int n(); int n(int); enum { n }; // accepted!

[llvmbot]

@llvm/issue-subscribers-c-1

[AaronBallman]

The issue still reproduces as of Clang 16: https://godbolt.org/z/TEMqxrhxn

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[AaronBallman]

Confirming as the example in the summary is a bug, but this behavior for LookupSingleName is the documented behavior for it: https://github.com/llvm/llvm-project/blob/2cbc265cc947c40372b841f80649276fbf9d183f/clang/include/clang/Sema/Sema.h#L4465