[scan-build] Pass through all -f and -O flags, along with -Wwrite-strings.

These flags control language options and user-visible macros, so it's
important to preserve them when analyzing. Rather than try to keep up
with all the -f flags, we'll pass them all through and then ban the ones
we don't want (like -fsyntax-only).

-Wwrite-strings is really an f-flag in disguise: it implies -fconst-strings.

Patch by Keaton Mowry, modified by me.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186138 91177308-0d34-0410-b5e6-96231b3b80d8