Provide API for identifying macros which are defined without a value

[PathogenDavid]

IE:

#define __LITERALLY_EVERY_HEADER_GUARD_H__

Right now these appear in the macros listing, and when they are evaluated they yield no value and no diagnostics.

We should also look into why there's no diagnostics because that ~~seems~~is definitely wrong. (Checked on Godbolt, Clang yields an error as expected. Seems to associate just fine too.)

Side note: "Value" probably isn't the right word here. Tokens? Body?