[DFSan] Remove an overly aggressive assert reported in PR26068.

This code has been successfully used to bootstrap libc++ in a no-asserts
mode for a very long time, so the code that follows cannot be completely
incorrect. I've added a test that shows the current behavior for this
kind of code with DFSan. If it is desirable for DFSan to do something
special when processing an invoke of a variadic function, it can be
added, but we shouldn't keep an assert that we've been ignoring due to
release builds anyways.

llvm-svn: 262829