[flang] Disable LTO when building the flang runtime

[DavidTruby]

Summary

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.

This patch disables LTO when building the flang runtime to avoid
this issue.

[DavidTruby]

Note: this doesn't actually make LTO builds work correctly. There appears to be a bug where LTO builds sometimes cause segmentation faults at flang runtime. This patch just allows LTO builds to build correctly.

[tblah]

LGTM, thanks for fixing this!

flang/lib/Decimal/CMakeLists.txt

if (IS_BIGENDIAN)
     add_compile_definitions(FLANG_BIG_ENDIAN=1)
   else ()
     add_compile_definitions(FLANG_LITTLE_ENDIAN=1)
   endif ()
   include_directories(BEFORE
     ${FLANG_SOURCE_DIR}/include)
 endif()
 
+check_cxx_compiler_flag(-fno-lto FLANG_RUNTIME_HAS_FNO_LTO_FLAG)
+if (FLANG_RUNTIME_HAS_FNO_LTO_FLAG)
+  append("-fno-lto" CMAKE_CXX_FLAGS)
+endif()
+
+
+
 add_flang_library(FortranDecimal INSTALL_WITH_TOOLCHAIN
   binary-to-decimal.cpp
   decimal-to-binary.cpp
 )

flang/runtime/CMakeLists.txt

check_cxx_source_compiles(
   "#include <string.h>
    int main() {
      char buf[4096];
      return strerror_s(buf, 4096, 0);
    }
   "
   HAVE_DECL_STRERROR_S)
 
+check_cxx_compiler_flag(-fno-lto FLANG_RUNTIME_HAS_FNO_LTO_FLAG)
+if (FLANG_RUNTIME_HAS_FNO_LTO_FLAG)
+  set(NO_LTO_FLAGS "-fno-lto")
+else()
+  set(NO_LTO_FLAGS "")
+endif()
+
 if (NOT (HAVE_STRERROR OR HAVE_STRERROR_R OR HAVE_DECL_STRERROR_S))
   message(FATAL_ERROR "None of strerror, strerror_r, strerror_s found.")
 endif()
 
 configure_file(config.h.cmake config.h)
 # include_directories is used here instead of target_include_directories
 # because add_flang_library creates multiple objects (STATIC/SHARED, OBJECT)
 # with different names
 include_directories(AFTER ${CMAKE_CURRENT_BINARY_DIR})
 
+append(${NO_LTO_FLAGS} CMAKE_C_FLAGS)
+append(${NO_LTO_FLAGS} CMAKE_CXX_FLAGS)
+
 add_subdirectory(FortranMain)
 
 add_flang_library(FortranRuntime
   ISO_Fortran_binding.cpp
   allocatable.cpp
   assign.cpp
   buffer.cpp
   command.cpp