Added VerifyDecl, a function that, when LLDB is

linked against a debug LLVM, runs a variety of
functions -- currently just one -- that verify
that the Decls we create are valid.

ClangASTContext now calls this verifier whenever
it adds a Decl to a DeclContext, and the verifier
checks that the AccessSpecifier is sane.

llvm-svn: 143000