[Sema] Reject value-initialization of function types

T in the expression T() must be a non-array complete object type or
the void type.  Function types are neither.

This fixes PR24798.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247535 91177308-0d34-0410-b5e6-96231b3b80d8