Update build infrastructure for LLVM 10.0.0.

ok hackroom@
tested by plenty

zap empty depends/silence empty installs.
okay millert@

Fix "make depend".

ok jsg@

LLVM/Clang build system. This only builds the bare minimum of what is needed
to get a working "clang" binary.

Note that you need GCC from ports to bootstrap clang.

looks good to patrick@, ok jsg@

LLVM/Clang build system. This only builds the bare minimum of what is needed
to get a working "clang" binary.

Note that you need GCC from ports to bootstrap clang.

looks good to patrick@, ok jsg@ kettenis@

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