Overhaul checking of non-type template arguments that should refer to

Overhaul checking of non-type template arguments that should refer to
an object or function. Our previous checking was too lax, and ended up
allowing missing or extraneous address-of operators, among other
evils. The new checking provides better diagnostics and adheres more
closely to the standard.

Fixes PR6563 and PR6749.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@100125 91177308-0d34-0410-b5e6-96231b3b80d8