Fixed the way the ClangExpressionDeclMap looks
up variables in the current stack frame to avoid
mutual recursion between the expression parser
and the synthetic child providers.  Variables
should only be looked up in a very simple way,
using no synthetic anything.

<rdar://problem/13173454>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@174947 91177308-0d34-0410-b5e6-96231b3b80d8