[CMake] Remove `CMAKE_.*_OUTPUT_DIRECTORY` (NFCI)

[modocache]

Summary

Three CMAKE_.*_OUTPUT_DIRECTORY variables used to be set in CMake and
referenced in various other parts of the project. However, in r198205
chapuni added a note to "don't set them anymore", and any remaining
references to them were subsequently removed in r198316 and r199592.

Now that the variables are no longer used anywhere, remove them, along
with the comments advising against using them any longer.

Test Plan:
I ran check-all and confirmed the tests built and passed.

[beanz]

LGTM. Please watch the bots closely with this change because CMake can be used in strange ways.

[modocache]

Thanks @beanz! I'll keep an eye on the bots. Here goes nothing! :)

[modocache]

It turns out polly makes use of these variables. I'll revert this for now and try again after taking a closer look at polly.

[modocache]

Reverted in https://reviews.llvm.org/rL314551.

CMakeLists.txt

 configure_file(
   ${LLVM_SRPM_USER_BINARY_SPECFILE}
   ${LLVM_SRPM_BINARY_SPECFILE} @ONLY)
 
 add_custom_target(srpm
   COMMAND cpack -G TGZ --config CPackSourceConfig.cmake -B ${LLVM_SRPM_DIR}/SOURCES
   COMMAND rpmbuild -bs --define '_topdir ${LLVM_SRPM_DIR}' ${LLVM_SRPM_BINARY_SPECFILE})
 
-
-# They are not referenced. See set_output_directory().
-set( CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/bin )
-set( CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
-set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
-
 if(APPLE AND DARWIN_LTO_LIBRARY)
   set(CMAKE_EXE_LINKER_FLAGS
     "${CMAKE_EXE_LINKER_FLAGS} -Wl,-lto_library -Wl,${DARWIN_LTO_LIBRARY}")
   set(CMAKE_SHARED_LINKER_FLAGS
     "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-lto_library -Wl,${DARWIN_LTO_LIBRARY}")
   set(CMAKE_MODULE_LINKER_FLAGS
     "${CMAKE_MODULE_LINKER_FLAGS} -Wl,-lto_library -Wl,${DARWIN_LTO_LIBRARY}")
 endif()

cmake/modules/AddLLVM.cmake

if(NOT LLVM_NO_DEAD_STRIP)
         set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                      LINK_FLAGS " -Wl,--gc-sections")
       endif()
     endif()
   endif()
 endfunction(add_link_opts)
 
 # Set each output directory according to ${CMAKE_CONFIGURATION_TYPES}.
-# Note: Don't set variables CMAKE_*_OUTPUT_DIRECTORY any more,
-# or a certain builder, for eaxample, msbuild.exe, would be confused.
 function(set_output_directory target)
   cmake_parse_arguments(ARG "" "BINARY_DIR;LIBRARY_DIR" "" ${ARGN})
 
   # module_dir -- corresponding to LIBRARY_OUTPUT_DIRECTORY.
   # It affects output of add_library(MODULE).
   if(WIN32 OR CYGWIN)
     # DLL platform
     set(module_dir ${ARG_BINARY_DIR})