Fix for issue 1440 (flang-compiler#180) (flang-compiler#187)

Through this commit Mbackslash will be the default option.
By default, escape characters are treated as regular characters unless Mnobackslash
is specified.

There is also the issue of treating both Mbackslash and Mnobackslash options independently.
This is addressed in this patch and the testcases in flang repository will test this feature.

clang/lib/Driver/ToolChains/ClassicFlang.cpp

@@ -240,22 +240,35 @@ void ClassicFlang::ConstructJob(Compilation &C, const JobAction &JA,
     }
   }
 
-  // Treat backslashes as regular characters
-  for (auto Arg : Args.filtered(options::OPT_fno_backslash, options::OPT_Mbackslash)) {
-    Arg->claim();
+  if (auto *A =
+          Args.getLastArg(options::OPT_fno_backslash, options::OPT_Mbackslash,
+                          options::OPT_fbackslash, options::OPT_Mnobackslash)) {
+    for (auto Arg :
+         Args.filtered(options::OPT_fno_backslash, options::OPT_Mbackslash,
+                       options::OPT_fbackslash, options::OPT_Mnobackslash)) {
+      Arg->claim();
+    }
+    // Treat backslashes as regular characters
+    if (A->getOption().matches(options::OPT_fno_backslash) ||
+        A->getOption().matches(options::OPT_Mbackslash)) {
+      CommonCmdArgs.push_back("-x");
+      CommonCmdArgs.push_back("124");
+      CommonCmdArgs.push_back("0x40");
+    }
+    // Treat backslashes as C-style escape characters
+    if (A->getOption().matches(options::OPT_fbackslash) ||
+        A->getOption().matches(options::OPT_Mnobackslash)) {
+      CommonCmdArgs.push_back("-y");
+      CommonCmdArgs.push_back("124");
+      CommonCmdArgs.push_back("0x40");
+    }
+  } else {
+    // By default treat backslashes as regular characters
     CommonCmdArgs.push_back("-x");
     CommonCmdArgs.push_back("124");
     CommonCmdArgs.push_back("0x40");
   }
 
-  // Treat backslashes as C-style escape characters
-  for (auto Arg : Args.filtered(options::OPT_fbackslash, options::OPT_Mnobackslash)) {
-    Arg->claim();
-    CommonCmdArgs.push_back("-y");
-    CommonCmdArgs.push_back("124");
-    CommonCmdArgs.push_back("0x40");
-  }
-
   // handle OpemMP options
   if (auto *A = Args.getLastArg(options::OPT_mp, options::OPT_nomp,
                              options::OPT_fopenmp, options::OPT_fno_openmp)) {