Remove the `CustomEntry` escape hatch from builtin TableGen (#120861)

This was an especially challenging escape hatch because it directly
forced the use of a specific X-macro structure and prevented any other
form of TableGen emission.

The problematic feature that motivated this is a case where a builtin's
prototype can't be represented in the mini-language used by TableGen.
Instead of adding a complete custom entry for this, this PR just teaches
the prototype handling to do the same thing the X-macros did in this
case: emit an empty string and let the Clang builtin handling respond
appropriately.

This should produce identical results while preserving all the rest of
the structured representation in the builtin TableGen code.

0 files changed, 0 insertions, 0 deletions