[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/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp b/lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp
index 69e59bc..08e3737 100644
--- a/lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp
+++ b/lldb/source/Plugins/SystemRuntime/MacOSX/SystemRuntimeMacOSX.cpp
@@ -412,7 +412,7 @@
 #endif
 
     ClangASTContext *ast_ctx =
-        m_process->GetTarget().GetScratchClangASTContext();
+        ClangASTContext::GetScratch(m_process->GetTarget());
     if (ast_ctx->getASTContext() &&
         m_dispatch_tsd_indexes_addr != LLDB_INVALID_ADDRESS) {
       CompilerType uint16 =