Remove an assert that's not true on invalid code.

Remove an assert that's not true on invalid code.

r185773 added an assert that checked that a CXXUnresolvedConstructExpr either
has a valid rparen, or exactly one argument.  This doesn't have to be true for
invalid inputs.  Convert the assert to an if, and add a test for this case.

Found by SLi's afl bot.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@225140 91177308-0d34-0410-b5e6-96231b3b80d8