devel/qt6-tools: Avoid LLVM version mismatches

In 6.9.0, the Tools component of Qt introduced more restrictive checks
for versions of LLVM it supports and automatically selects the newest
version installed without an offical override option. This is
problematic for us, because if a user has a version of LLVM installed
greater than what they have set in DEFAULT_VERSIONS, it will cause
configuration errors due to CMake mismatching the latest version of LLVM
found with the version we actually want to build against.

Patch to only supply the version of LLVM we intend to build against to
the checks. These checks need at least the major and minor version of
LLVM to work. Unfortunately, LLVM_VERSION, which only provide the major
version isn't enough, so there's some silliness which is hopefully
explained well enough in the Makefile and in the patch for future
hackers.

PR: 287430
Reported by: Ivan Rozhuk <rozhuk.im@gmail.com>, Chad Jacob Milios <milios@ccsys.com>