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