[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purpose was to support that build mode, and some outdated documentation.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- Update some outdated documentation referencing standalone builds

cmake/Modules/HandleOutOfTreeLLVM.cmake

@@ -1,80 +0,0 @@
-if (NOT DEFINED LLVM_PATH)
-  set(LLVM_PATH ${CMAKE_CURRENT_LIST_DIR}/../../llvm CACHE PATH "" FORCE)
-endif()
-
-if(NOT IS_DIRECTORY ${LLVM_PATH})
-  message(FATAL_ERROR
-    "The provided LLVM_PATH (${LLVM_PATH}) is not a valid directory. Note that "
-    "building libc++ outside of the monorepo is not supported anymore. Please "
-    "use a Standalone build against the monorepo, a Runtimes build or a classic "
-    "monorepo build.")
-endif()
-
-set(LLVM_INCLUDE_DIR ${LLVM_PATH}/include CACHE PATH "Path to llvm/include")
-set(LLVM_PATH ${LLVM_PATH} CACHE PATH "Path to LLVM source tree")
-set(LLVM_MAIN_SRC_DIR ${LLVM_PATH})
-set(LLVM_CMAKE_DIR "${LLVM_PATH}/cmake/modules")
-set(LLVM_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
-set(LLVM_LIBRARY_OUTPUT_INTDIR "${CMAKE_CURRENT_BINARY_DIR}/lib")
-
-if (EXISTS "${LLVM_CMAKE_DIR}")
-  list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_DIR}")
-elseif (EXISTS "${LLVM_MAIN_SRC_DIR}/cmake/modules")
-  list(APPEND CMAKE_MODULE_PATH "${LLVM_MAIN_SRC_DIR}/cmake/modules")
-else()
-  message(FATAL_ERROR "Neither ${LLVM_CMAKE_DIR} nor ${LLVM_MAIN_SRC_DIR}/cmake/modules found. "
-                      "This is not a supported configuration.")
-endif()
-
-message(STATUS "Configuring for standalone build.")
-
-# By default, we target the host, but this can be overridden at CMake invocation time.
-include(GetHostTriple)
-get_host_triple(LLVM_INFERRED_HOST_TRIPLE)
-set(LLVM_HOST_TRIPLE "${LLVM_INFERRED_HOST_TRIPLE}" CACHE STRING "Host on which LLVM binaries will run")
-set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_HOST_TRIPLE}" CACHE STRING "Target triple used by default.")
-
-# Add LLVM Functions --------------------------------------------------------
-if (WIN32)
-  set(LLVM_ON_UNIX 0)
-  set(LLVM_ON_WIN32 1)
-else()
-  set(LLVM_ON_UNIX 1)
-  set(LLVM_ON_WIN32 0)
-endif()
-
-include(AddLLVM OPTIONAL)
-
-# LLVM Options --------------------------------------------------------------
-if (NOT DEFINED LLVM_INCLUDE_TESTS)
-  set(LLVM_INCLUDE_TESTS ON)
-endif()
-if (NOT DEFINED LLVM_INCLUDE_DOCS)
-  set(LLVM_INCLUDE_DOCS ON)
-endif()
-if (NOT DEFINED LLVM_ENABLE_SPHINX)
-  set(LLVM_ENABLE_SPHINX OFF)
-endif()
-
-if (LLVM_INCLUDE_TESTS)
-  # Required LIT Configuration ------------------------------------------------
-  # Define the default arguments to use with 'lit', and an option for the user
-  # to override.
-  set(LLVM_DEFAULT_EXTERNAL_LIT "${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py")
-  set(LIT_ARGS_DEFAULT "-sv --show-xfail --show-unsupported")
-  if (MSVC OR XCODE)
-    set(LIT_ARGS_DEFAULT "${LIT_ARGS_DEFAULT} --no-progress-bar")
-  endif()
-  set(LLVM_LIT_ARGS "${LIT_ARGS_DEFAULT}" CACHE STRING "Default options for lit")
-endif()
-
-# Required doc configuration
-if (LLVM_ENABLE_SPHINX)
-  find_package(Sphinx REQUIRED)
-endif()
-
-if (LLVM_ON_UNIX AND NOT APPLE)
-  set(LLVM_HAVE_LINK_VERSION_SCRIPT 1)
-else()
-  set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
-endif()