[cmake] Remove obsolete files, docs and CMake variables related to the standalone build (#112741)

The runtimes used to support a build mode called the "Standalone build",
which isn't supported anymore (and hasn't been for a few years).
However, various places in the code still contained stuff whose only
purpose was to support that build mode, and some outdated documentation.
This patch cleans that up (although I probably missed some).

- Remove HandleOutOfTreeLLVM.cmake which isn't referenced anymore
- Remove the LLVM_PATH CMake variable which isn't used anymore
- Update some outdated documentation referencing standalone builds

GitOrigin-RevId: 6127724786d581d2653df7420d1f28709288a107

docs/BuildingLibunwind.rst

diff --git a/docs/BuildingLibunwind.rst b/docs/BuildingLibunwind.rst
index 79166b4..8b4f120 100644
--- a/docs/BuildingLibunwind.rst
+++ b/docs/BuildingLibunwind.rst
@@ -16,7 +16,7 @@
 However if you want to build tip-of-trunk from here (getting the bleeding
 edge), read on.
 
-The basic steps needed to build libc++ are:
+The basic steps needed to build libunwind are:
 
 #. Checkout LLVM, libunwind, and related projects:
 
@@ -50,23 +50,6 @@
    * ``make install-unwind`` --- Will install the libraries and the headers
 
 
-It is sometimes beneficial to build outside of the LLVM tree. An out-of-tree
-build would look like this:
-
-.. code-block:: bash
-
-  $ cd where-you-want-libunwind-to-live
-  $ # Check out llvm, and libunwind
-  $ ``svn co https://llvm.org/svn/llvm-project/llvm/trunk llvm``
-  $ ``svn co https://llvm.org/svn/llvm-project/libunwind/trunk libunwind``
-  $ cd where-you-want-to-build
-  $ mkdir build && cd build
-  $ export CC=clang CXX=clang++
-  $ cmake -DLLVM_PATH=path/to/llvm \
-          path/to/libunwind
-  $ make
-
-
 .. _CMake Options:
 
 CMake Options