Don't use #_sourceLocation in the fallback event handler.

[grynspan]

We can't use our own macros in the Testing target because they aren't available when we build the toolchain.

Update the expansion of #_sourceLocation to detect it's being used in one of our own library targets and emit an error.

Resolves rdar://171774246.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[harlanhaskins]

Clever use of compiler conditions to prevent this bug in the future, nice

[stmontgomery]

(For tracking purposes) Fallout from #1523

[finagolfin]

I just ran into this build error when trying to build latest trunk locally on macOS, thought maybe I did something wrong. 😄 May help to set up some kind of nightly snapshot CI using build-script to build this repo, as that would likely have turned up this bug during PR testing.

[grynspan]

I just ran into this build error when trying to build latest trunk locally on macOS, thought maybe I did something wrong. 😄 May help to set up some kind of nightly snapshot CI using build-script to build this repo, as that would likely have turned up this bug during PR testing.

I'd have to defer to our CI wranglers on that topic.