devel/wasi-{compiler-rt,libcxx}: attempt to unbreak the tree

In the meta-ports, USES=llvm is defined before IGNORE, so when
DISTVERSION=0, they are marked BROKEN because LLVM version 0 is
invalid. However, the USES=llvm alone will permeate ${LLVM_DEFAULT}
unless a valid specific LLVM version/range is specified, so anything
that references the USES=llvm variables will defer to ${LLVM_DEFAULT}.
In these cases, the children of these meta-ports are split by
individual LLVM version, referencing them specifically.

Each devel/wasi-libcxx depends on the corresponding devel/wasi-compiler-rt
using ${LLVM_VERSION} through the respective meta-ports. However,
since the meta-port is not IGNOREd until after the invalid USES=llvm,
${LLVM_VERSION} is ${LLVM_DEFAULT}, which is still as of this commit
15, one of the dropped versions in WASI. BUILD_DEPENDS in the
meta-port as a result refers to a removed port, breaking the tree.

Conditionalise USES=llvm until after the proper IGNORE to avoid
this kind of situation now and into the future.

Reported by: antoine