[driver] [analyzer] Fix a backward compatibility issue after r348038.

Since r348038 we emit an error every time an -analyzer-config option is not
found. The driver, however, suppresses this error with another flag,
-analyzer-config-compatibility-mode, so backwards compatibility is maintained,
while analyzer developers still enjoy the new typo-free experience.

The backwards compatibility turns out to be still broken when the -analyze
action is not specified; it is still possible to specify -analyzer-config
in that case. This should be fixed now.

Patch by Kristóf Umann!

Differential Revision: https://reviews.llvm.org/D55823

rdar://problem/46504165

llvm-svn: 349824