[CUDA] Use only the GVALinkage on function definitions.

Summary:
Previously we'd look at the GVALinkage of whatever FunctionDecl you
happened to be calling.

This is not right.  In the absence of the gnu_inline attribute, to be
handled separately, the function definition determines the function's
linkage.  So we need to wait until we get a def before we can know
whether something is known-emitted.

Reviewers: tra

Subscribers: cfe-commits, rsmith

Differential Revision: https://reviews.llvm.org/D26268

llvm-svn: 286313