[analyzer] Don't run non-path-sensitive checks on system headers...

...but do run them on user headers.

Previously, we were inconsistent here: non-path-sensitive checks on code
/bodies/ were only run in the main source file, but checks on
/declarations/ were run in /all/ headers. Neither of those is the
behavior we want.

Thanks to Sujit for pointing this out!

<rdar://problem/12454226>

llvm-svn: 165635