Load custom plugins when running scan-build. This is useful when
additional static analysis Checkers must be provided via clang's plugin
interface.

Loading additional plugins can now be done via the scan-build call:
scan-build -load-plugin <plugin.so>

A patch by Thomas Hauth.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157452 91177308-0d34-0410-b5e6-96231b3b80d8