[cmake] Don't pass -z discard-unused to Illumos ld

[ro]

Summary

I'm currently working to port libc++ to Solaris. There exists a slightly bitrotten port
already, which was done on Illumos, an OpenSolaris derivative. In order not to break that
port with my work, I need to test the result on both Solaris and Illumos. While doing so,
it turned out that Illumos ld doesn't support the -z discard-sections=unused option
currently used on SunOS unconditionally.

While there exists a patch
for LLVM 9.0 in the OpenIndiana repository, it apparently hasn't been submitted
upstream and is completely wrong: it replaces -z discard-sections=unused with
-z ignore. In terms of the equivalent gld options, this means replacing --gc-sections
with --as-needed.

This patch instead tests if the linker actually supports the option before using it.

Tested on amd64-pc-solaris2.11 (all of Solaris 11.4, 11.3 and OpenIndiana 2020.04).

Ok for master?

[stephenneuendorffer]

Seems reasonable to me.

llvm/cmake/modules/AddLLVM.cmake

if (NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
     endif()
 
     if(NOT LLVM_NO_DEAD_STRIP)
       if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
         # ld64's implementation of -dead_strip breaks tools that use plugins.
         set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                      LINK_FLAGS " -Wl,-dead_strip")
       elseif(${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
+        # Support for ld -z discard-unused=sections was only added in
+        # Solaris 11.4.
+        include(CheckLinkerFlag)
+        check_linker_flag("-Wl,-z,discard-unused=sections" LINKER_SUPPORTS_Z_DISCARD_UNUSED)
+        if (LINKER_SUPPORTS_Z_DISCARD_UNUSED)
           set_property(TARGET ${target_name} APPEND_STRING PROPERTY
-                     LINK_FLAGS " -Wl,-z -Wl,discard-unused=sections")
+                       LINK_FLAGS " -Wl,-z,discard-unused=sections")
+        endif()
       elseif(NOT WIN32 AND NOT LLVM_LINKER_IS_GOLD AND
              NOT ${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD|AIX")
         # Object files are compiled with -ffunction-data-sections.
         # Versions of bfd ld < 2.23.1 have a bug in --gc-sections that breaks
         # tools that use plugins. Always pass --gc-sections once we require
         # a newer linker.
         set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                      LINK_FLAGS " -Wl,--gc-sections")