Where clause parsing does not gracefully exit in case of an invalid decl

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#1096

---

If we have code like this:

void f() {
  _Nt_array_ptr<char> p : count(0);
}

The compiler emits an error:

a.c:4: error: automatic variable 'p' with bounds must have initializer
  _Nt_array_ptr<char> p : count(0);

However, if a where clause is attached to this invalid decl:

void f() {
  _Nt_array_ptr<char> p : count(0) _Where 1 == 1;
}

The compiler runs into an infinite loop and emits a lot of these error messages:

a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
a.c:4:44: error: where clause on invalid decl
...