[flang][driver]Fix broken flang-new mlir test

[Leporacanthicus]

Summary

The test was added as a .mlir file, and this extension is not
in the lit.cfg.py, so it was never run. When running it, the
file would produce an error, as semicolon is not an MLIR comment.

This adds the extension and fixes the comment start by using C++
style comments.

[awarzynski]

Thanks Mats!

[awarzynski]

flang/test/lit.cfg.py

31

[nit] Perhaps .mlir and .fir should be next to each other?

flang/test/Driver/emit-asm-from-mlir.mlir

-; Verify that the driver can consume MLIR/FIR files.
+// Verify that the driver can consume MLIR/FIR files.
 
-; RUN: %flang_fc1 -S %s -o - | FileCheck %s
-
-; CHECK-LABEL: foo:
-; CHECK: ret
+// RUN: %flang_fc1 -S %s -o - | FileCheck %s
 
+// CHECK-LABEL: foo:
+// CHECK: ret
 func.func @foo() {
   return
 }

flang/test/lit.cfg.py

 # For now we require '&&' between commands, until they get globally killed and
 # the test runner updated.
 config.test_format = lit.formats.ShTest(not llvm_config.use_lit_shell)
 
 # suffixes: A list of file extensions to treat as test files.
 config.suffixes = ['.c', '.cpp', '.f', '.F', '.ff', '.FOR', '.for', '.f77', '.f90', '.F90',
                    '.ff90', '.f95', '.F95', '.ff95', '.fpp', '.FPP', '.cuf'
                    '.CUF', '.f18', '.F18', '.fir', '.f03', '.F03', '.f08',
-                   '.F08', '.ll']
+                   '.F08', '.ll', '.mlir']
 
 config.substitutions.append(('%PATH%', config.environment['PATH']))
 config.substitutions.append(('%llvmshlibdir', config.llvm_shlib_dir))
 config.substitutions.append(('%pluginext', config.llvm_plugin_ext))
 
 llvm_config.use_default_substitutions()
 
 # ask llvm-config about asserts