Instead of using COMPILER_RT_INSTALL_PATH through the CMake for
complier-rt, just use it to define variables for the subdirs which
themselves are used.
This preserves compatibility, but later on we might consider getting rid
of COMPILER_RT_INSTALL_PATH and just changing the defaults for the
subdir variables directly.
There was a seaming bug where the (non-Apple) per-target libdir was
${target} not lib/${target}. I suspect that has to do with the docs
on COMPILER_RT_INSTALL_PATH saying was the library dir when that's no
longer true, so I just went ahead and fixed it, allowing me to define
fewer and more sensible variables.
That last part should be the only behavior changes; everything else
should be a pure refactoring.
D99484 is the main thrust of the GnuInstallDirs work. Once this lands,
it should be feasible to follow both of these up with a simple patch for
compiler-rt analogous to the one for libcxx.
The suspicious, out-of-date description I described in the commit message.