build-llvm: Bump to LLVM 19.1.0 RC 1

The main changes for mingw targets since 18.x is primarily
minor bug fixes. llvm-dlltool has also been improved to handle
cases of renamed imports, where the target of the rename doesn't
exist as a normal import.

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-18.1.8}
+: ${LLVM_VERSION:=llvmorg-19.1.0-rc1}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"