Skip over AnnotatedLines with >50 levels of nesting; don't format them.

Skip over AnnotatedLines with >50 levels of nesting; don't format them.

Reasoning:
- ExpressionParser uses a lot of stack for these, bad in some environments.
- Our formatting algorithm is N^3 and gets really slow.
- The resulting formatting is unlikely to be any good.
- This is probably generated code we're formatting by accident.

We treat these as unparseable, and signal incomplete formatting. 50 is
an arbitrary number, I've only seen real problems from ~150 levels.

Patch by Sam McCall. Thank you.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285570 91177308-0d34-0410-b5e6-96231b3b80d8