[c++17] When deducing the type of a non-type template parameter from the type

[c++17] When deducing the type of a non-type template parameter from the type
of its argument, perform function-to-pointer and array-to-pointer decay on the
parameter type first.

Otherwise deduction will fail, as the type of the argument will be decayed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319584 91177308-0d34-0410-b5e6-96231b3b80d8