Fix treatment of case which came up on std-proposals@: 'void' is permitted in...

Fix treatment of case which came up on std-proposals@: 'void' is permitted in core constant expressions, despite not being a literal type.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164968 91177308-0d34-0410-b5e6-96231b3b80d8