Added AnalysisConsumer, a meta-level ASTConsumer class to drive various

analyses. This potentially is the primordial origins of a Clang-equivalent
"PassManager".

The new AnalysisConsumer interface allows multiple analyses to be run from a
single invocation of Clang.

Migrated the logic of "-warn-dead-stores" and "-warn-uninit-values" to use the
new AnalysisConsumer interface. The new interface results in a significant code
reduction to incorporate an analysis into the Driver.

Updated a test case to (correctly) acknowledge that it contains a dead store
(this check wasn't being performed because it was previously masked by
-warn-uninit-values).

llvm-svn: 52996