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

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.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D133792

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

@@ -1,10 +1,9 @@
-; 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

@@ -27,8 +27,8 @@ 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']
+                   '.CUF', '.f18', '.F18', '.f03', '.F03', '.f08', '.F08',
+                   '.ll', '.fir', '.mlir']
 
 config.substitutions.append(('%PATH%', config.environment['PATH']))
 config.substitutions.append(('%llvmshlibdir', config.llvm_shlib_dir))