[libc++] Support per-target __config_site in per-target runtime build

When using the per-target runtime build, it may be desirable to have
different __config_site headers for each target where all t

[libc++] Support per-target __config_site in per-target runtime build

When using the per-target runtime build, it may be desirable to have
different __config_site headers for each target where all targets cannot
share a single configuration.

The layout used for libc++ headers after this change is:

```
include/
c++/
v1/
<libc++ headers except for __config_site>
<target1>/
c++/
v1/
__config_site
<target2>/
c++/
v1/
__config_site
<other targets>
```

This is the most optimal layout since it avoids duplication, the only
headers that's per-target is __config_site, all other headers are
shared across targets. This also means that we no need two
-isystem flags: one for the target-agnostic headers and one for
the target specific headers.

Differential Revision: https://reviews.llvm.org/D89013

show more ...