Sema: Implement DR317

Summary:
Declaring a function as inline after it has been defined is in violation
of [dcl.fct.spec]p4.  The program would get a strong definition instead
of getting a function with linkonce_odr linkage.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3220