Add a "Potentially Breaking Changes" section to the Clang release notes

Sometimes we make changes to the compiler that we expect may cause
disruption for users. For example, we may strengthen a warning to
default to be an error, or fix an accepts-invalid bug that's been
around for a long time, etc which may cause previously accepted code to
now be rejected. Rather than hope users discover that information by
reading all of the release notes, it's better that we call these out in
one location at the top of the release notes.

Based on feedback collected in the discussion at:
https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213/

Differential Revision: https://reviews.llvm.org/D133771