[llvm] prefix linker flag on non-MSVC compilers with -Wl,

Prior to this patch, a Windows build of llvm-lto using clang failed with
the error: `LTO.def: unknown file type`.  The reason for this failure is
that .DEF files are used by the linker not by the clang compiler.  The
MSVC compiler+linker handles this transparently, but if we're using
clang (or gcc), then we need to tell the compiler to forward this flag
to the linker. This patch adds the necessary `-Wl` flag to fix the
problem.

Reviewed By: rnk, mstorsjo

Differential Revision: https://reviews.llvm.org/D134165

Author: ashay

llvm/cmake/modules/AddLLVM.cmake

@@ -118,7 +118,7 @@ function(add_llvm_symbol_exports target_name export_file)
       set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                    LINK_FLAGS "  -Wl,--version-script,\"${CMAKE_CURRENT_BINARY_DIR}/${native_export_file}\"")
     endif()
-  else()
+  elseif(WIN32)
     set(native_export_file "${target_name}.def")
 
     add_custom_command(OUTPUT ${native_export_file}
@@ -129,7 +129,18 @@ function(add_llvm_symbol_exports target_name export_file)
       COMMENT "Creating export file for ${target_name}")
     set(export_file_linker_flag "${CMAKE_CURRENT_BINARY_DIR}/${native_export_file}")
     if(MSVC)
+      # cl.exe or clang-cl, i.e. MSVC style command line interface
       set(export_file_linker_flag "/DEF:\"${export_file_linker_flag}\"")
+    elseif(CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
+      # clang in msvc mode, calling a link.exe/lld-link style linker
+      set(export_file_linker_flag "-Wl,/DEF:\"${export_file_linker_flag}\"")
+    elseif(MINGW)
+      # ${export_file_linker_flag}, which is the plain file name, works as is
+      # when passed to the compiler driver, which then passes it on to the
+      # linker as an input file.
+      set(export_file_linker_flag "\"${export_file_linker_flag}\"")
+    else()
+      message(FATAL_ERROR "Unsupported Windows toolchain")
     endif()
     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                  LINK_FLAGS " ${export_file_linker_flag}")