Use the naming class from the overloaded lookup when access-checking an

Use the naming class from the overloaded lookup when access-checking an
address of overloaded function, instead of assuming that a nested name
specifier was used.  A nested name specifier is not required for static
functions.

Fixes PR6886.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102107 91177308-0d34-0410-b5e6-96231b3b80d8