framework: Add new USES=llvm to more easily depend on one an llvm port

USES=llvm supports two kinds of arguments:

• mode

<none>: defaults to build
build: add a BUILD_DEPENDS on chosen llvm-port
run: add a RUN_DEPENDS on chosen llvm-port
lib: add a LIB_DEPENDS on chosen llvm-port

• version

<none>: defaults to ${LLVM_DEFAULT}
number: use explicitly the specified version
min=number: use specified min if ${LLVM_DEFAULT} is lower
max=number: use specified max if ${LLVM_DEFAULT} is higher

An example usage might be:
USES= llvm

or

USES= llvm:13,build

or

USES= llvm:min=14,lib

The following variables are exposed to be used in the ports tree at the moment:

• LLVM_VERSION version chosen from the arguments to llvm.mk
• LLVM_PORT chosen llvm port
• LLVM_CONFIG llvm-config of the chosen port
• LLVM_LIBLLVM libLLVM.so of the chosen port
• LLVM_PREFIX installation prefix of the chosen port

Reviewed by: manu, bapt
Differential Revision: https://reviews.freebsd.org/D37728