Added "CheckASTConsumer", a function that generalizes

"CheckDiagnostics" (used for -parse-ast-check) to check the
diagnostics of any ASTConsumer.

Reimplemented CheckDiagnostics to use CheckASTConsumer instead.

Added driver option -warn-dead-stores-check, which checks the
diagnostics generated by the DeadStores checker.  This is implemented
using CheckASTConsumer.111

llvm-svn: 42310