[cmake] Support specifying the libdir suffix for runtimes separately

[mgorny]

Summary

Add LLVM_RUNTIMES_LIBDIR_SUFFIX cache variable to allow configuring
the libdir suffix for runtimes separately, and store its value in
LLVMConfig.

This change originates from D23752 which aimed to add the support for
this override in clang. The goal is to support sharing a common runtime
directory between multiple multilib variants of LLVM/clang.

For example, on a multilib amd64 system the 64-bit and 32-bit versions
of clang libraries could both use the same runtime. This is possible
because runtime consists of arch-independent headers and libraries that
are arch-suffixed already.

[mgorny]

Since @chandlerc rejected the clang part, this makes no sense anymore.

CMakeLists.txt

 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 
 if (CMAKE_BUILD_TYPE AND
     NOT uppercase_CMAKE_BUILD_TYPE MATCHES "^(DEBUG|RELEASE|RELWITHDEBINFO|MINSIZEREL)$")
   message(FATAL_ERROR "Invalid value for CMAKE_BUILD_TYPE: ${CMAKE_BUILD_TYPE}")
 endif()
 
 set(LLVM_LIBDIR_SUFFIX "" CACHE STRING "Define suffix of library directory name (32/64)" )
+set(LLVM_RUNTIMES_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
+  "Define suffix of library directory used to hold runtimes (32/64)")
 
 set(LLVM_TOOLS_INSTALL_DIR "bin" CACHE STRING "Path for binary subdirectory (defaults to 'bin')")
 mark_as_advanced(LLVM_TOOLS_INSTALL_DIR)
 
 # They are used as destination of target generators.
 set(LLVM_RUNTIME_OUTPUT_INTDIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin)
 set(LLVM_LIBRARY_OUTPUT_INTDIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX})
 if(WIN32 OR CYGWIN)

cmake/modules/LLVMConfig.cmake.in

 set(LLVM_ENABLE_PLUGINS @LLVM_ENABLE_PLUGINS@)
 set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS @LLVM_EXPORT_SYMBOLS_FOR_PLUGINS@)
 set(LLVM_PLUGIN_EXT @LLVM_PLUGIN_EXT@)
 
 set(LLVM_ON_UNIX @LLVM_ON_UNIX@)
 set(LLVM_ON_WIN32 @LLVM_ON_WIN32@)
 
 set(LLVM_LIBDIR_SUFFIX @LLVM_LIBDIR_SUFFIX@)
+set(LLVM_RUNTIMES_LIBDIR_SUFFIX @LLVM_RUNTIMES_LIBDIR_SUFFIX@)
 
 set(LLVM_INCLUDE_DIRS "@LLVM_CONFIG_INCLUDE_DIRS@")
 set(LLVM_LIBRARY_DIRS "@LLVM_CONFIG_LIBRARY_DIRS@")
 
 # These variables are duplicated, but they must match the LLVM variables of the
 # same name. The variables ending in "S" could some day become lists, and are
 # preserved for convention and compatibility.
 set(LLVM_INCLUDE_DIR "@LLVM_CONFIG_INCLUDE_DIRS@")

docs/CMake.rst

**CMAKE_INSTALL_PREFIX**:PATH
   Path where LLVM will be installed if "make install" is invoked or the
   "install" target is built.
 
 **LLVM_LIBDIR_SUFFIX**:STRING
   Extra suffix to append to the directory where libraries are to be
   installed. On a 64-bit architecture, one could use ``-DLLVM_LIBDIR_SUFFIX=64``
   to install libraries to ``/usr/lib64``.
 
+**LLVM_RUNTIMES_LIBDIR_SUFFIX**:STRING
+  Extra suffix to append to the library directory where LLVM runtimes
+  are to be installed. Defaults to LLVM_LIBDIR_SUFFIX. One can override
+  it to share a single runtime directory between 64-bit and 32-bit LLVM
+  builds.
+
 **CMAKE_C_FLAGS**:STRING
   Extra flags to use when compiling C source files.
 
 **CMAKE_CXX_FLAGS**:STRING
   Extra flags to use when compiling C++ source files.
 
 .. _LLVM-specific variables: