add feature to support for LLVM 19

[tw-ilson]

Added support for llvm-19 major version, which will allow melior to bind to LLVM C API build from source to test changes.

[edg-l]

llvm 19 isnt out yet, so i wouldnt make it the default feature yet

[tw-ilson]

Here's my test results:

tblgen-rs-alt git:master ❯ cargo t --no-default-features --features llvm19-0 --tests                                                                                                                                                         ⏎
   Compiling tblgen-alt v0.3.6 (/Users/tw/Developer/tblgen-rs-alt)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 1.55s
     Running unittests src/lib.rs (target/debug/deps/tblgen_alt-a09caa9395542a5e)

running 14 tests
test raw::bindgen_test_layout_TableGenStringRef ... ok
test init::tests::bits ... ok
test init::tests::bit ... ok
test init::tests::int ... ok
test init::tests::dag ... ok
test init::tests::list ... ok
test init::tests::string ... ok
test record::tests::record ... ok
test record::tests::print_error ... ok
test record::tests::values ... ok
test record_keeper::test::classes_and_defs ... ok
test record_keeper::test::derived_defs ... ok
test record_keeper::test::single ... ok
test record::tests::single_value ... ok

test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s