Disable LTO

Apply https://reviews.llvm.org/D140016 to LLVM_ENABLE_RUNTIMES=flang-rt

Author: Meinersbur

flang-rt/CMakeLists.txt

@@ -204,6 +204,9 @@ endif ()
 # but some older versions of CMake don't define it for GCC itself.
 check_cxx_compiler_flag("-UTESTFLAG" FLANG_RT_SUPPORTS_UNDEFINE_FLAG)
 
+# Check whether -fno-lto is supported.
+check_cxx_compiler_flag(-fno-lto FLANG_RT_HAS_FNO_LTO_FLAG)
+
 
 # function checks
 find_package(Backtrace)

flang-rt/cmake/modules/AddFlangRT.cmake

@@ -126,6 +126,13 @@ function (add_flangrt_library name)
     target_compile_options(${name} PUBLIC -U_LIBCPP_ENABLE_ASSERTIONS)
   endif ()
 
+  # When building the flang runtime if LTO is enabled the archive file
+  # contains LLVM IR rather than object code. Currently flang is not
+  # LTO aware so cannot link this file to compiled Fortran code.
+  if (FLANG_RT_HAS_FNO_LTO_FLAG)
+    target_compile_options(${name} PRIVATE -fno-lto)
+  endif ()
+
   # Flang/Clang (including clang-cl) -compiled programs targeting the MSVC ABI
   # should only depend on msvcrt/ucrt. LLVM still emits libgcc/compiler-rt
   # functions in some cases like 128-bit integer math (__udivti3, __modti3,