[[Clang CallGraph]] CallGraph should still record calls to decls.

Discovered by a downstream user, we found that the CallGraph ignores
callees unless they are defined.  This seems foolish, and prevents
combining the report with other reports to create unified reports.
Additionally, declarations contain information that is likely useful to
consumers of the CallGraph.

This patch implements this by splitting the includeInGraph function into
two versions, the current one plus one that is for callees only.  The
only difference currently is that includeInGraph checks for a body, then
calls includeCalleeInGraph.

Differential Revision: https://reviews.llvm.org/D76435