Don't crash when generating code for __attribute__((naked)) member functions.

Summary:
Previously this crashed inside EmitThisParam().  There should be no
prelude for naked functions, so just skip the whole thing.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22715

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276925 91177308-0d34-0410-b5e6-96231b3b80d8