Allow empty statements in naked functions in addition to ASM statements

Summary: This fixes PR20883.

Test Plan: The patch includes an automated test.

Reviewers: hansw

Differential Revision: http://reviews.llvm.org/D5256

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@217413 91177308-0d34-0410-b5e6-96231b3b80d8