[analyzer] Treat analyzer-synthesized function bodies like implicit bodies.

[analyzer] Treat analyzer-synthesized function bodies like implicit bodies.

When generating path notes, implicit function bodies are shown at the call
site, so that, say, copying a POD type in C++ doesn't jump you to a header
file. This is especially important when the synthesized function itself
calls another function (or block), in which case we should try to jump the
user around as little as possible.

By checking whether a called function has a body in the AST, we can tell
if the analyzer synthesized the body, and if we should therefore collapse
the call down to the call site like a true implicitly-defined function.

<rdar://problem/13978414>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182677 91177308-0d34-0410-b5e6-96231b3b80d8