Change Parser::ParseCaseStatement to use an iterative approach to parsing

Change Parser::ParseCaseStatement to use an iterative approach to parsing
multiple sequential case statements instead of doing it with recursion.  This
fixes a problem where we run out of stack space parsing 100K directly nested
cases.

There are a couple other problems that prevent this from being useful in
practice (right now the example only parses correctly with -disable-free and
doesn't work with -emit-llvm), but this is a start.

I'm not including a testcase because it is large and uninteresting for
regtesting.

Sebastian, I would appreciate it if you could scrutinize the smart pointer
gymnastics I do.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@66011 91177308-0d34-0410-b5e6-96231b3b80d8