[driver] [analyzer] Fix --analyze -Xanalyzer after r349863.

If an -analyzer-config is passed through -Xanalyzer, it is not found while
looking for -Xclang.

Additionally, don't emit -analyzer-config-compatibility-mode for *every*
-analyzer-config flag we encounter; one is enough.

https://reviews.llvm.org/D55823

rdar://problem/46504165

llvm-svn: 349866