[analyzer] Remove an assertion that doesn't hold in C++17.

Function return values can be constructed directly in variables or passed
directly into return statements, without even an elidable copy in between.
This is how the C++17 mandatory copy elision AST behaves. The behavior we'll
have in such cases is the "old" behavior that we've had before we've
implemented destructor inlining and proper lifetime extension support.

Differential Revision: https://reviews.llvm.org/D44755

llvm-svn: 328253