[lldb][windows] Fix crash on getting nested exception

LLDB tries to follow `EXCEPTION_RECORD::ExceptionRecord` to follow the
nested exception chain. In practice this code just causes Access
Violation whenever there is a nested exception. Since there does not
appear to be any code in LLDB that is actually using the nested
exceptions, this change just removes the crashing code and adds a
comment for future reference.

Fixes https://github.com/mstorsjo/llvm-mingw/issues/292

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D128201