build-llvm: Bump to LLVM 18.1.1

The 18.1.0 release accidentally had kept the "rc" suffix on the
version. The 18.1.1 release fixes this, no other changes from 18.1.0.

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-18.1.0}
+: ${LLVM_VERSION:=llvmorg-18.1.1}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"