[runtimes] Revert the libc++ __config_site change

This is a massive revert of the following commits (from most revent to oldest):

	2b9b7b5775a1d8fcd7aa5abaa8fc0bc303434f1a.
	529ac33197f6408952ae995075ac5e2dc5287e81
	28270234f1478047e35879f4ba8838b47edfcc14
	69c2087283cf7b17ca75f69daebf4ffc158b754a
	b5aa67446e01bd277727b05710a42e69ac41e74b
	5d796645d6c8cadeb003715c33e231a8ba05b6de

After checking-in the __config_site change, a lot of things started breaking
due to widespread reliance on various aspects of libc++'s build, notably the
fact that we can include the headers from the source tree, but also reliance
on various "internal" CMake variables used by the runtimes build and compiler-rt.

These were unintended consequences of the change, and after two days, we
still haven't restored all the bots to being green. Instead, now that I
understand what specific areas this will blow up in, I should be able to
chop up the patch into smaller ones that are easier to digest.

See https://reviews.llvm.org/D89041 for more details on this adventure.

GitOrigin-RevId: 48e4b0fd3a3d68cc9774699964cf4c6c2be38cf3
diff --git a/docs/TestingLibcxx.rst b/docs/TestingLibcxx.rst
index ec017e2..d42becf 100644
--- a/docs/TestingLibcxx.rst
+++ b/docs/TestingLibcxx.rst
@@ -26,8 +26,8 @@
 
 After building libc++, you can run parts of the libc++ test suite by simply
 running ``llvm-lit`` on a specified test or directory. If you're unsure
-whether the required targets have been built, you can use the `check-cxx-deps`
-target to build them. For example:
+whether the required libraries have been built, you can use the
+`check-cxx-deps` target. For example:
 
 .. code-block:: bash
 
@@ -37,12 +37,6 @@
   $ <build>/bin/llvm-lit -sv libcxx/test/std/depr/depr.c.headers/stdlib_h.pass.cpp # Run a single test
   $ <build>/bin/llvm-lit -sv libcxx/test/std/atomics libcxx/test/std/threads # Test std::thread and std::atomic
 
-In the default configuration, the tests are built against headers that form a
-fake installation root of libc++. This installation root has to be updated when
-changes are made to the headers, so you should re-run the `check-cxx-deps` target
-before running the tests manually with `lit` when you make any sort of change,
-including to the headers.
-
 Sometimes you'll want to change the way LIT is running the tests. Custom options
 can be specified using the `--param=<name>=<val>` flag. The most common option
 you'll want to change is the standard dialect (ie -std=c++XX). By default the