[Driver] Ignore -fno-semantic-interposition

Fedora wants to build projects with -fno-semantic-interposition (e.g.
https://fedoraproject.org/wiki/Changes/PythonNoSemanticInterpositionSpeedup),
which is supported by GCC>=5.

Clang's current behavior is similar to -fno-semantic-interposition and
the end goal is to make it more so
(https://lists.llvm.org/pipermail/llvm-dev/2016-November/107625.html).
Ignore this option.

We should let users know -fsemantic-interposition is not currently
supported, so it should remain a hard error.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D72724