[libclc] Make library output directories explicit (#146833)

These changes were split off from #146503.

This commit makes the output directories of libclc artefacts explicit.
It creates a variable for the final output directory -
LIBCLC_OUTPUT_LIBRARY_DIR - which has not changed. This allows future
changes to alter the output directory more simply, such as by pointing
it to somewhere inside clang's resource directory.

This commit also changes the output directory of each target's
intermediate builtins.*.bc files. They are now placed into each
respective libclc target's object directory, rather than the top-level
libclc binary directory. This should help keep the binary directory a
bit tidier.

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