[TEST] Improve tests for #pragma clang optimize off/on

Added coverage for:
* More than one "off region" in the same file
* An "off region" falling off the end of an included file



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214086 91177308-0d34-0410-b5e6-96231b3b80d8