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purpose was to support that build mode, and some outdated documentation.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- Update some outdated documentation referencing standalone builds

runtimes/CMakeLists.txt

@@ -75,9 +75,6 @@ set(LLVM_RUNTIME_OUTPUT_INTDIR ${LLVM_TOOLS_BINARY_DIR})
 set(LLVM_MAIN_SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 set(LLVM_CMAKE_DIR ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 
-# This variable is used by individual runtimes to locate LLVM files.
-set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
-
 include(CheckLibraryExists)
 include(LLVMCheckCompilerLinkerFlag)
 include(CheckCCompilerFlag)