Fixed an assertion failure triggered by invalid code.

Set invalid type of declarator after emitting error diagnostics,
so that it won't be later considered when instantiating the template.
Added test5_inst in test/SemaCXX/condition.cpp for non-regression.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172201 91177308-0d34-0410-b5e6-96231b3b80d8