[analyzer] Explain why we have system-header-simulator*.h files.

Suggested by Csaba. Text based on an e-mail of mine on cfe-dev.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174213 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed