Implement name lookup for conversion function template specializations

Implement name lookup for conversion function template specializations
(C++ [temp.mem]p5-6), which involves template argument deduction based
on the type named, e.g., given

  struct X { template<typename T> operator T*(); } x;

when we call

  x.operator int*();

we perform template argument deduction to determine that T=int. This
template argument deduction is needed for template specialization and
explicit instantiation, e.g.,

  template<> X::operator float*() { /* ... */ }

and when calling or otherwise naming a conversion function (as in the
first example).

This fixes PR5742 and PR5762, although there's some remaining ugliness
that's causing out-of-line definitions of conversion function
templates to fail. I'll look into that separately.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@93162 91177308-0d34-0410-b5e6-96231b3b80d8