[llvm] [llvm][Tablegen][llvm-tblgen] Added keyword #undef to llvm-tblgen (PR #69135)

[zhao jiangtao via llvm-commits]

whousemyname wrote:

> I kind of follow what you're saying, but it's not clear to me why this is a good structure or the problem it really solves (that is I see the "made up" structure you described, but haven't seen a strong need for this in the real world so far). Having three different files for the conceptually three different thing to include should be fine here.

My starting point is that in addition to the possibility of using the undef function in my own project described above, another  starting point is that since there is #define, #undef may also be needed. 
### I will close this PR and thank all reviewers again for their review.

https://github.com/llvm/llvm-project/pull/69135