Change LLVMConventionsChecker to accept an entire translation unit instead

Change LLVMConventionsChecker to accept an entire translation unit instead
of operating on each code decl.  This exposes two flaws in AnalysisConsumer
that should eventually be fixed:

(1) It is not possible to associate multiple "actions" with a single
    command line argument.  This will require the notion of an
	"analysis" group, and possibly tablegen support.  (although eventually
    we want to support dynamically loading analyses as well)

(2) AnalysisConsumer may not actually be scanning the declarations in namespaces.
    We'll experiment first in LLVMConventionsChecker before changing the
    behavior in AnalysisConsumer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@96183 91177308-0d34-0410-b5e6-96231b3b80d8