[flang] Rename `flang` as `flang-to-external-fc`

[awarzynski]

Summary

For context, see:
https://discourse.llvm.org/t/rename-flang-new-as-flang.

Our ultimate goal is to rename flang-new as flang. This is "step 1"
in which we just rename the bash script. flang becomes a symbolic link
pointing to flang-to-external-fc. This will allow a smooth transmission
for any buildbots that do use this script.

[DavidSpickett]

LGTM. Please check https://reviews.llvm.org/D125796 looks correct and I will land that after this, get the bot updated and you can go ahead with the 2nd part.

[awarzynski]

Change flang-to-gfortran to flang-to-external-fc

Suggested by @kiranchandramohan in https://reviews.llvm.org/D125788

[awarzynski]

This change was effectively agreed on in https://reviews.llvm.org/D125788 and hence I merged it without waiting for more reviews.

flang/tools/f18/CMakeLists.txt

foreach(filename ${MODULES})
   install(FILES ${base}.mod ${base}.f18.mod DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/flang")
 endforeach()
 
 add_custom_target(module_files ALL DEPENDS ${MODULE_FILES})
 
 # This flang shell script will only work in a POSIX shell.
 if (NOT WIN32)
   add_custom_command(
-    OUTPUT ${CMAKE_BINARY_DIR}/bin/flang
-    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/flang
-    COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/flang ${CMAKE_BINARY_DIR}/bin)
-  add_custom_target(flang ALL DEPENDS ${CMAKE_BINARY_DIR}/bin/flang)
-  install(PROGRAMS ${CMAKE_BINARY_DIR}/bin/flang DESTINATION "${CMAKE_INSTALL_BINDIR}")
+    OUTPUT ${CMAKE_BINARY_DIR}/bin/flang-to-external-fc
+    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/flang-to-external-fc
+    COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/flang-to-external-fc ${CMAKE_BINARY_DIR}/bin)
+  add_custom_target(flang-to-external-fc ALL DEPENDS ${CMAKE_BINARY_DIR}/bin/flang-to-external-fc)
+  install(PROGRAMS ${CMAKE_BINARY_DIR}/bin/flang-to-external-fc DESTINATION "${CMAKE_INSTALL_BINDIR}")
 endif()
+add_custom_target(flang-slink ALL DEPENDS ${CMAKE_BINARY_DIR}/bin/flang-to-external-fc COMMAND ${CMAKE_COMMAND} -E create_symlink
+  ${CMAKE_BINARY_DIR}/bin/flang-to-external-fc ${CMAKE_BINARY_DIR}/bin/flang)
 
 # TODO Move this to a more suitable location
 file(COPY ${FLANG_SOURCE_DIR}/module/omp_lib.h DESTINATION "${CMAKE_BINARY_DIR}/include/flang/OpenMP/" FILE_PERMISSIONS OWNER_READ OWNER_WRITE)