Fix PR40495 - is_invokable_v<void> does not compile

The meta-programming that attempted to form the invoke call expression
was not in a SFINAE context. This made it a hard error to provide
non-referencable types like 'void' or 'void (...) const'.

This patch fixes the error by checking the validity of the call
expression within a SFINAE context.

llvm-svn: 352522