Flang: Add support for flang Fortran compiler

flang is a Fortran compiler built on top of clang [1].  Because flang
shares a lot of commonalities with clang, the flang module piggybacks
off the clang module and overrides certain options.

Add flang to Fortran compiler auto find list.

Update flang preprocessor macros to differentiate from PGI.

Add Flang-FindBinUtils.

[1] https://github.com/flang-compiler/flang

Help/release/dev/flang.rst

@@ -0,0 +1,6 @@
+flang
+-----
+
+* The `flang`_ Fortran compiler is now supported, with compiler id ``Flang``.
+
+.. _`flang`: https://github.com/flang-compiler/flang

Help/variable/CMAKE_LANG_COMPILER_ID.rst

@@ -27,6 +27,7 @@ include:
   NVIDIA = NVIDIA CUDA Compiler (nvidia.com)
   OpenWatcom = Open Watcom (openwatcom.org)
   PGI = The Portland Group (pgroup.com)
+  Flang = Flang Fortran Compiler
   PathScale = PathScale (pathscale.com)
   SDCC = Small Device C Compiler (sdcc.sourceforge.net)
   SunPro = Oracle Solaris Studio (oracle.com)

Modules/CMakeDetermineFortranCompiler.cmake

@@ -52,6 +52,7 @@ else()
       #  frt: Fujitsu F77 compiler
       #  pathf90/pathf95/pathf2003: PathScale Fortran compiler
       #  pgf77/pgf90/pgf95/pgfortran: Portland Group F77/F90/F95 compilers
+      #  flang: Flang Fortran compiler
       #  xlf/xlf90/xlf95: IBM (AIX) F77/F90/F95 compilers
       #  lf95: Lahey-Fujitsu F95 compiler
       #  fl32: Microsoft Fortran 77 "PowerStation" compiler
@@ -68,7 +69,7 @@ else()
       set(CMAKE_Fortran_COMPILER_LIST
         ftn
         ifort ifc af95 af90 efc f95 pathf2003 pathf95 pgf95 pgfortran lf95 xlf95
-        fort gfortran gfortran-4 g95 f90 pathf90 pgf90 xlf90 epcf90 fort77
+        fort flang gfortran gfortran-4 g95 f90 pathf90 pgf90 xlf90 epcf90 fort77
         frt pgf77 xlf fl32 af77 g77 f77 nag
         )
 
@@ -77,10 +78,11 @@ else()
       set(_Fortran_COMPILER_NAMES_Intel     ifort ifc efc)
       set(_Fortran_COMPILER_NAMES_Absoft    af95 af90 af77)
       set(_Fortran_COMPILER_NAMES_PGI       pgf95 pgfortran pgf90 pgf77)
+      set(_Fortran_COMPILER_NAMES_Flang     flang)
       set(_Fortran_COMPILER_NAMES_PathScale pathf2003 pathf95 pathf90)
       set(_Fortran_COMPILER_NAMES_XL        xlf)
       set(_Fortran_COMPILER_NAMES_VisualAge xlf95 xlf90 xlf)
-      set(_Fortran_COMPILER_NAMES_NAG nagfor)
+      set(_Fortran_COMPILER_NAMES_NAG       nagfor)
     endif()
 
     _cmake_find_compiler(Fortran)

Modules/CMakeFortranCompilerId.F.in

@@ -90,6 +90,13 @@
 # if defined(__PGIC_PATCHLEVEL__)
 #  define COMPILER_VERSION_PATCH DEC(__PGIC_PATCHLEVEL__)
 # endif
+#elif defined(__FLANG)
+        PRINT *, 'INFO:compiler[Flang]'
+# define COMPILER_VERSION_MAJOR DEC(__FLANG_MAJOR__)
+# define COMPILER_VERSION_MINOR DEC(__FLANG_MINOR__)
+# if defined(__FLANG_PATCHLEVEL__)
+#  define COMPILER_VERSION_PATCH DEC(__FLANG_PATCHLEVEL__)
+# endif
 #elif defined(_SGI_COMPILER_VERSION) || defined(_COMPILER_VERSION)
         PRINT *, 'INFO:compiler[MIPSpro]'
 #       if 0

Modules/Compiler/Flang-FindBinUtils.cmake

@@ -0,0 +1 @@
+include(Compiler/Clang-FindBinUtils)

Modules/Compiler/Flang-Fortran.cmake

@@ -0,0 +1,12 @@
+include(Compiler/Clang)
+__compiler_clang(Fortran)
+
+set(CMAKE_Fortran_PREPROCESS_SOURCE
+    "<CMAKE_Fortran_COMPILER> -cpp <DEFINES> <INCLUDES> <FLAGS> -E <SOURCE> -o <PREPROCESSED_SOURCE>")
+
+set(CMAKE_Fortran_FORMAT_FIXED_FLAG "-ffixed-form")
+set(CMAKE_Fortran_FORMAT_FREE_FLAG "-ffree-form")
+
+string(APPEND CMAKE_Fortran_FLAGS_DEBUG_INIT " -fbounds-check")
+
+set(CMAKE_Fortran_MODDIR_FLAG "-J")