[analyzer] Make inclusion/exclusion of checkers less ambiguous.

A checker may be enabled/disabled multiple times via -enable-checker and -disable-checker scan-build arguments. Currently the conflicting and repetitive arguments are passed to the analyzer as is.
With this patch only the last enable/disable of a particular checker is accepted and passed to the analyzer.
This change is mostly done for the upcoming 'config for scan-build' patch when multiple inclusions/exclusions of a checker are expected to be more common.

llvm-svn: 251524