devel/cquery: Allow overriding the LLVM version via LLVM_DEFAULT

• Cquery builds and works fine with LLVM 7 for example
• Make the libclang.so spec more specific to prevent the framework from picking the wrong library version from llvm60 with LLVM_DEFAULT=70 when both llvm60 and llvm70 are installed in the build environment

PR: 232598
Approved by: henry.hu.sh@gmail.com (maintainer timeout, 2 weeks)