address of an overloaded function (or function template), perform that
resolution prior to determining the implicit conversion
sequence. This resolution is not part of the implicit conversion
sequence itself.

Previously, we would always consider this resolution to be a
function pointer decay, which was a lie: there might be an explicit &
in the expression, in which case decay should not occur. This caused
the CodeGen assertion in PR6973 (where we created a 
pointer to a pointer to a function when we should have had a pointer
to a function), but it's likely that there are corner cases of
overload resolution where this would have failed.

Cleaned up the code involved in determining the type that will
produced afer resolving the overloaded function reference, and added
an assertion to make sure the result is correct. Fixes PR6973.

llvm-svn: 102650