[Build] Detect ld.gold version and prefer lld if gold is too old.

If we're on a system that has ld.gold 2.35 or earlier, we want to use
lld instead because otherwise we end up with duplicate sections in the
output.

rdar://123504095

Author: al45tair

CMakeLists.txt

@@ -975,6 +975,8 @@ endif()
 # Which default linker to use. Prefer LLVM_USE_LINKER if it set, otherwise use
 # our own defaults. This should only be possible in a unified (not stand alone)
 # build environment.
+include(GoldVersion)
+
 if(LLVM_USE_LINKER)
   set(SWIFT_USE_LINKER_default "${LLVM_USE_LINKER}")
 elseif(SWIFT_HOST_VARIANT_SDK STREQUAL "ANDROID")
@@ -984,7 +986,17 @@ elseif(CMAKE_SYSTEM_NAME STREQUAL "Windows" AND NOT CMAKE_HOST_SYSTEM_NAME STREQ
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
   set(SWIFT_USE_LINKER_default "")
 else()
-  set(SWIFT_USE_LINKER_default "gold")
+  get_gold_version(gold_version)
+  if(NOT gold_version)
+    message(STATUS "GNU Gold not found; using lld instead")
+    set(SWIFT_USE_LINKER_default "lld")
+  elseif(gold_version VERSION_LESS "2.36")
+    message(STATUS "GNU Gold is too old (${gold_version}); using lld instead")
+    set(SWIFT_USE_LINKER_default "lld")
+  else()
+    message(STATUS "Using GNU Gold ${gold_version}")
+    set(SWIFT_USE_LINKER_default "gold")
+  endif()
 endif()
 set(SWIFT_USE_LINKER ${SWIFT_USE_LINKER_default} CACHE STRING
     "Build Swift with a non-default linker")

cmake/modules/GoldVersion.cmake

@@ -0,0 +1,18 @@
+# Find the version of ld.gold, if installed.
+#
+# Versions prior to 2.36 break Swift programs because they won't coalesce
+# sections with different SHF_GNU_RETAIN flags.
+function(get_gold_version result_var_name)
+  find_program(gold_executable "ld.gold")
+  if(gold_executable)
+    execute_process(
+      COMMAND "${gold_executable}" "--version"
+      COMMAND "head" "-n" "1"
+      COMMAND "sed" "-e" "s/^.* (\\([^)]*\\)).*$/\\1/g;s/.* \\([0-9][0-9]*\\(\\.[0-9][0-9]*\\)*\\).*/\\1/g"
+      OUTPUT_VARIABLE gold_version
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+    set("${result_var_name}" "${gold_version}" PARENT_SCOPE)
+  else()
+    set("${result_var_name}" "" PARENT_SCOPE)
+  endif()
+endfunction()