Revert "[AST] Enable AST node introspection on Apple"

This reverts commit 907409a5.

This caused a failure

  http://green.lab.llvm.org/green//job/lldb-cmake-standalone/2827/consoleFull#-210109660a1ca8a51-895e-46c6-af87-ce24fa4cd561

Assertion failed: (!CodeSynthesisContexts.empty() && "Cannot perform an
instantiation without some context on the " "instantiation stack"),
function SubstType, file
/Users/buildslave/jenkins/workspace/lldb-cmake-standalone/llvm-project/clang/lib/Sema/SemaTemplateInstantiate.cpp,
line 2071.