[add changelog entry]

Author: kinke

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 #### Big news
 - Frontend, druntime and Phobos are at version ~[2.112.0](https://dlang.org/changelog/2.112.0.html), incl. new command-line options `-extI`, `-dllimport=externalOnly` and `-edition`. (#4949, #4962, #4988, #5029)
+- Support for [LLVM 21](https://releases.llvm.org/21.1.0/docs/ReleaseNotes.html). Beware that patch versions before v21.1.8 seem to misoptimize `std.json`. (#4950, #5033)
 - **Breaking change for dcompute**: The special `@kernel` UDA is now a function and _**requires**_ parentheses as in `@kernel() void foo(){}`. Optionally you can provide launch dimensions, `@kernel([2,4,8])`, to specify to the compute runtime how the kernel is intended to be launched.
 - ldc2.conf can now be a directory. All the files inside it, ordered naturally, will be concatenated and treated like a big config. (#4954)
   - Running `ldc-build-runtime --installWithSuffix` now includes installing a target-specific .conf file to that directory. (#4978)
@@ -15,6 +16,7 @@
 - The prebuilt arm64/universal macOS packages additionally bundle the arm64 iOS-*simulator* libraries, for out-of-the-box cross-compilation support via e.g. `-mtriple=arm64-apple-ios12.0-simulator`. (#4974)
 
 #### Platform support
+- Supports LLVM 15 - 21.
 
 #### Bug fixes