[AST][Introspection] Add a check to detect if introspection is supported.

This could probably be made into a compile time constant, but that would involve generating a second inc file.

Reviewed By: steveire

Differential Revision: https://reviews.llvm.org/D100530

GitOrigin-RevId: f019e5f73ed732b374e376f5ddbba5d1f67dca0c
4 files changed