[CUDA] [AST] Allow isInlineDefinitionExternallyVisible to be called on functions without bodies.

Summary:
In CUDA compilation, we call isInlineDefinitionExternallyVisible (via
getGVALinkageForFunction) on functions while parsing their definitions.

At the point in time when we call getGVALinkageForFunction, we haven't
yet added the body to the function, so we trip this assert.  But as far
as I can tell, this is harmless.

To work around this, we add a new flag to FunctionDecl, "WillHaveBody".

There was other code that was working around the existing assert with a
really awful hack -- this change lets us get rid of that hack.

Reviewers: rsmith, tra

Subscribers: aemerson, cfe-commits

Differential Revision: https://reviews.llvm.org/D25640

llvm-svn: 285410