[analyzer] Fix out-of-tree only clang build by not relaying on private header

It turned out that the D78704 included a private LLVM header, which is excluded
from the LLVM install target.
I'm substituting that `#include` with the public one by moving the necessary
`#define` into that. There was a discussion about this at D78704 and on the
cfe-dev mailing list.

I'm also placing a note to remind others of this pitfall.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D84929