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

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
<https://github.com/OpenIndiana/oi-userland/blob/oi/hipster/components/developer/clang-90/patches/02-cmake_modules_AddLLVM.cmake.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).

Differential Revision: https://reviews.llvm.org/D81545

llvm/cmake/modules/AddLLVM.cmake

@@ -240,8 +240,14 @@ function(add_link_opts target_name)
         set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                      LINK_FLAGS " -Wl,-dead_strip")
       elseif(${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
-        set_property(TARGET ${target_name} APPEND_STRING PROPERTY
-                     LINK_FLAGS " -Wl,-z -Wl,discard-unused=sections")
+        # 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,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.