GCC compatibility: Create a Group to ignore unsupported optimization.

Returns a warning when using an unknown optimization flag.
This patch includes -finline-limit as one of those ignored flags.
More options will be moved in this group

Patch by Arthur Marble <arthur@info9.net> in the context of
Debian Google Summer of code 2014.

Reviewers: rnk, Sylvestre



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212805 91177308-0d34-0410-b5e6-96231b3b80d8