[Lex] Add support for 'user specified system frameworks' (see test case).
 - Developers of system frameworks need a way for their framework to be treated as a "system framework" during development. Otherwise, they are unable to properly test how their framework behaves when installed because of the semantic changes (in warning behavior) applied to system frameworks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154105 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed