Flang: Fix command-line used to preprocess sources

Running flang with `-E` now ignores any `-o` option and always prints
preprocessed output to stdout.  Use shell redirection to place it in a
file instead.

Fixes: #18497

Modules/Compiler/Flang-Fortran.cmake

@@ -2,7 +2,7 @@ include(Compiler/Clang)
 __compiler_clang(Fortran)
 
 set(CMAKE_Fortran_PREPROCESS_SOURCE
-    "<CMAKE_Fortran_COMPILER> -cpp <DEFINES> <INCLUDES> <FLAGS> -E <SOURCE> -o <PREPROCESSED_SOURCE>")
+    "<CMAKE_Fortran_COMPILER> -cpp <DEFINES> <INCLUDES> <FLAGS> -E <SOURCE> > <PREPROCESSED_SOURCE>")
 
 set(CMAKE_Fortran_FORMAT_FIXED_FLAG "-ffixed-form")
 set(CMAKE_Fortran_FORMAT_FREE_FLAG "-ffree-form")