Update LLVM to 21.1.3
Fixes https://github.com/rust-lang/rust/issues/146742.
After rust-lang/rust#146124, we need more space to run x86_64-gnu-distcheck if building LLVM from source. According to the building log, the space freed by `free-disk-space-linux.sh` is not entirely available.
```
You are running out of disk space.
The runner will stop working when the machine runs out of disk space.
Free space left: 98 MB
disk usage:
Filesystem Size Used Avail Use% Mounted on
/dev/root 72G 43G 29G 60% /
tmpfs 7.9G 84K 7.9G 1% /dev/shm
tmpfs 3.2G 1.2M 3.2G 1% /run
tmpfs 5.0M 0 5.0M 0% /run/lock
/dev/sda16 881M 60M 760M 8% /boot
/dev/sda15 105M 6.2M 99M 6% /boot/efi
/dev/sdb1 74G 28K 70G 1% /mnt
tmpfs 1.6G 12K 1.6G 1% /run/user/1001
```
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