Added a function to check whether a Decl is in
the list of Decls for a given DeclContext.  This
is useful for LLDB's implementation of
FindExternalLexicalDecls.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181093 91177308-0d34-0410-b5e6-96231b3b80d8