community/qt5-qttools: upgrade _llvmver to 18

Fix this error:

https://build.alpinelinux.org/buildlogs/build-edge-loongarch64/community/qt5-qttools/qt5-qttools-5.15.10_git20230612-r1.log

WARNING: QDoc will not be compiled, probably because libclang could not be located. This means that you cannot build the Qt documentation.