Fix assertion failure in self-host (and probably bogus template instantiation

too). When instantiating a direct-initializer, if we find it has zero
arguments, produce an empty ParenListExpr rather than returning a null
expression.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170490 91177308-0d34-0410-b5e6-96231b3b80d8