[build] Add LLVM_LOCAL_RPATH which can set an rpath on just unit test binaries

[thakis]

Summary

After D80096, bots that build clang for distribution and that can't use system gcc / libstdc++ need to pass a working rpath so that unit test binaries can run. The method suggested in GettingStarted.rst works fine for local development, but it results in an absolute local rpath ending up even in distributed binaries like clang, which is both ugly and unnecessary.

Add an explicit toggle that can be used to add an rpath only for the non-distributed binaries that need it.

[hans]

Nice, thanks!

llvm/CMakeLists.txt

option(LLVM_ENABLE_STRICT_FIXED_SIZE_VECTORS
        "Enable assertions that type is not scalable in implicit conversion from TypeSize to uint64_t and calls to getNumElements" OFF)
 
 set(LLVM_ABI_BREAKING_CHECKS "WITH_ASSERTS" CACHE STRING
   "Enable abi-breaking checks.  Can be WITH_ASSERTS, FORCE_ON or FORCE_OFF.")
 
 option(LLVM_FORCE_USE_OLD_TOOLCHAIN
        "Set to ON to force using an old, unsupported host toolchain." OFF)
 
+set(LLVM_LOCAL_RPATH "" CACHE FILEPATH
+  "If set, an absolute path added as rpath on binaries that do not already contain an executable-relative rpath.")
+
 option(LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN
        "Set to ON to only warn when using a toolchain which is about to be deprecated, instead of emitting an error." OFF)
 
 option(LLVM_USE_INTEL_JITEVENTS
   "Use Intel JIT API to inform Intel(R) VTune(TM) Amplifier XE 2011 about JIT code"
   OFF)
 
 if( LLVM_USE_INTEL_JITEVENTS )

llvm/cmake/modules/AddLLVM.cmake

macro(add_llvm_executable name)
   else()
     add_executable(${name} ${ALL_FILES})
   endif()
 
   setup_dependency_debugging(${name} ${LLVM_COMMON_DEPENDS})
 
   if(NOT ARG_NO_INSTALL_RPATH)
     llvm_setup_rpath(${name})
+  elseif (LLVM_LOCAL_RPATH)
+    set_target_properties(${name} PROPERTIES
+                          BUILD_WITH_INSTALL_RPATH On
+                          INSTALL_RPATH "${LLVM_LOCAL_RPATH}")
   endif()
 
   if(DEFINED windows_resource_file)
     set_windows_version_resource_properties(${name} ${windows_resource_file})
   endif()
 
   # $<TARGET_OBJECTS> doesn't require compile flags.
   if(NOT LLVM_ENABLE_OBJLIB)

llvm/docs/GettingStarted.rst

.. code-block:: console
   % cd build
   % CC=$HOME/toolchains/bin/gcc CXX=$HOME/toolchains/bin/g++ \
     cmake .. -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$HOME/toolchains/lib64 -L$HOME/toolchains/lib64"
 
 If you fail to set rpath, most LLVM binaries will fail on startup with a message
 from the loader similar to ``libstdc++.so.6: version `GLIBCXX_3.4.20' not
 found``. This means you need to tweak the -rpath linker flag.
 
+This method will add an absolute path to the rpath of all executables. That's
+fine for local development. If you want to distribute the binaries you build
+so that they can run on older systems, copy ``libstdc++.so.6`` into the
+``lib/`` directory.  All of LLVM's shipping binaries have an rpath pointing at
+``$ORIGIN/../lib``, so they will find ``libstdc++.so.6`` there.  Non-distributed
+binaries don't have an rpath set and won't find ``libstdc++.so.6``. Pass
+``-DLLVM_LOCAL_RPATH="$HOME/toolchains/lib64"`` to cmake to add an absolute
+path to ``libstdc++.so.6`` as above. Since these binaries are not distributed,
+having an absolute local path is fine for them.
+
 When you build Clang, you will need to give *it* access to modern C++
 standard library in order to use it as your new host in part of a bootstrap.
 There are two easy ways to do this, either build (and install) libc++ along
 with Clang and then use it with the ``-stdlib=libc++`` compile and link flag,
 or install Clang into the same prefix (``$HOME/toolchains`` above) as GCC.
 Clang will look within its own prefix for libstdc++ and use it if found. You
 can also add an explicit prefix for Clang to look in for a GCC toolchain with
 the ``--gcc-toolchain=/opt/my/gcc/prefix`` flag, passing it to both compile and