Simplify AddSphinxTarget.cmake by just including `GNUInstallDirs`

[Ericson2314]

Summary

See the discussion in D30906 for how it must be robust to downstream
projects that might not use GNUInstallDirs.

I think an easier approach is just to include GNUInstallDirs so that
we can always rely on it. Looking at its definition in
https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/GNUInstallDirs.cmake
it appears safe to include multiple times.

I am trying this out because we will face the same problem many times
over if we use GNUInstallDirs more widely, as proposed in D99484 and
its parent revisions, since we do not know what downstream projects do
and rather not break them.

[phosek]

LGTM

[Ericson2314]

Rebase after conflict

llvm/cmake/modules/AddSphinxTarget.cmake

+include(GNUInstallDirs)
 
 # Create sphinx target
 if (LLVM_ENABLE_SPHINX)
   message(STATUS "Sphinx enabled.")
   find_package(Sphinx REQUIRED)
   if (LLVM_BUILD_DOCS AND NOT TARGET sphinx)
     add_custom_target(sphinx ALL)
   endif()
function (add_sphinx_target builder project)
   endif()
 
   if (LLVM_BUILD_DOCS)
     add_dependencies(sphinx ${SPHINX_TARGET_NAME})
 
     # Handle installation
     if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
       if (builder STREQUAL man)
-        if (CMAKE_INSTALL_MANDIR)
-          set(INSTALL_MANDIR "${CMAKE_INSTALL_MANDIR}/")
-        else()
-          set(INSTALL_MANDIR share/man/)
-        endif()
         # FIXME: We might not ship all the tools that these man pages describe
         install(DIRECTORY "${SPHINX_BUILD_DIR}/" # Slash indicates contents of
                 COMPONENT "${project}-sphinx-man"
-                DESTINATION ${INSTALL_MANDIR}man1)
+                DESTINATION "${CMAKE_INSTALL_MANDIR}/man1")
 
         if(NOT LLVM_ENABLE_IDE)
           add_llvm_install_targets("install-${SPHINX_TARGET_NAME}"
                                    DEPENDS ${SPHINX_TARGET_NAME}
                                    COMPONENT "${project}-sphinx-man")
         endif()
       elseif (builder STREQUAL html)
         string(TOUPPER "${project}" project_upper)