Add `-verify-ignore-unexpected` option to ignore unexpected diagnostics in VerifyDiagnosticsConsumer

Summary:
The goal of this patch is to make `-verify` easier to use when testing libc++. The `notes` attached to compile error diagnostics are numerous and relatively unstable when they reference libc++ header internals. This patch allows libc++ to write stable compilation failure tests by allowing unexpected diagnostic messages to be ignored where they are not relevant.

This patch adds a new CC1 flag called `-verify-ignore-unexpected`. `-verify-ignore-unexpected` tells `VerifyDiagnosticsConsumer` to ignore *all* unexpected diagnostic messages. `-verify-ignore-unexpected=<LevelList>` can be used to only ignore certain diagnostic levels. `<LevelList>` is a comma separated list of diagnostic levels to ignore. The supported levels are `note`, `remark`, `warning` and `error`.



Reviewers: bogner, grosser, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10138

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239665 91177308-0d34-0410-b5e6-96231b3b80d8