[Target] Remove Target::GetScratchClangASTContext

Target doesn't really need to know about ClangASTContext more than any
other TypeSystem. We can create a method ClangASTContext::GetScratch for
anything who needs a ClangASTContext specifically instead of just a
generic TypeSystem.
diff --git a/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderMacOS.cpp b/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderMacOS.cpp
index aab0e12..fd7d94f4 100644
--- a/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderMacOS.cpp
+++ b/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderMacOS.cpp
@@ -223,7 +223,10 @@
     // get the values from the ABI:
 
     ClangASTContext *clang_ast_context =
-        process->GetTarget().GetScratchClangASTContext();
+        ClangASTContext::GetScratch(process->GetTarget());
+    if (!clang_ast_context)
+      return false;
+
     ValueList argument_values;
 
     Value mode_value;    // enum dyld_notify_mode { dyld_notify_adding=0,