Update clang build infrastructure for LLVM 11.1.0.

Heavy lifting by mortimer@, thank you so much!

Update build infrastructure for LLVM 10.0.0.

ok hackroom@
tested by plenty

Build and install a shared libLLVM, llvm-config and llvm includes.
This is required to build the radeonsi Mesa driver.

ok patrick@

zap empty depends/silence empty installs.
okay millert@

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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