[clang][driver] Set the input type to Fortran when reading from stdin

This patch makes sure that for the following invocation of the new Flang
driver, clangDriver sets the input type to Fortran:
```
flang-new -E -
```
This change does not affect `clang`, i.e. for the following invocation
the input type is set to C:
```
clang -E -
```

This change leverages the fact that for `flang-new` the driver is in
Flang mode.

Differential Revision: https://reviews.llvm.org/D96777

Author: banach-space

clang/lib/Driver/Driver.cpp

@@ -2191,15 +2191,20 @@ void Driver::BuildInputs(const ToolChain &TC, DerivedArgList &Args,
 
         // stdin must be handled specially.
         if (memcmp(Value, "-", 2) == 0) {
-          // If running with -E, treat as a C input (this changes the builtin
-          // macros, for example). This may be overridden by -ObjC below.
-          //
-          // Otherwise emit an error but still use a valid type to avoid
-          // spurious errors (e.g., no inputs).
-          if (!Args.hasArgNoClaim(options::OPT_E) && !CCCIsCPP())
-            Diag(IsCLMode() ? clang::diag::err_drv_unknown_stdin_type_clang_cl
-                            : clang::diag::err_drv_unknown_stdin_type);
-          Ty = types::TY_C;
+          if (IsFlangMode()) {
+            Ty = types::TY_Fortran;
+          } else {
+            // If running with -E, treat as a C input (this changes the
+            // builtin macros, for example). This may be overridden by -ObjC
+            // below.
+            //
+            // Otherwise emit an error but still use a valid type to avoid
+            // spurious errors (e.g., no inputs).
+            if (!Args.hasArgNoClaim(options::OPT_E) && !CCCIsCPP())
+              Diag(IsCLMode() ? clang::diag::err_drv_unknown_stdin_type_clang_cl
+                              : clang::diag::err_drv_unknown_stdin_type);
+            Ty = types::TY_C;
+          }
         } else {
           // Otherwise lookup by extension.
           // Fallback is C if invoked as C preprocessor, C++ if invoked with

flang/test/Flang-Driver/input-from-stdin.f90

@@ -5,19 +5,22 @@
 !--------------------------
 ! FLANG DRIVER (flang-new)
 !--------------------------
-! TODO: Add support for `flang-new -`
-! Currently `bin/flang-new  -E -` defaults to `-x c` and e.g. F90 is not allowed
-! in `-x <input-type>` (see `clang::driver::types::canTypeBeUserSpecified` in
-! Types.cpp)
+! Input type is implicit
+! RUN: cat %s | flang-new -E - | FileCheck %s --check-prefix=PP-NOT-DEFINED
+! RUN: cat %s | flang-new -DNEW -E - | FileCheck %s --check-prefix=PP-DEFINED
+
+! Input type is explicit
+! RUN: cat %s | flang-new -E -x f95-cpp-input - | FileCheck %s --check-prefix=PP-NOT-DEFINED
+! RUN: cat %s | flang-new -DNEW -E -x f95-cpp-input - | FileCheck %s --check-prefix=PP-DEFINED
 
 !---------------------------------------
 ! FLANG FRONTEND DRIVER (flang-new -fc1)
 !---------------------------------------
-! Test `-E` - for the corresponding frontend actions the driver relies on the prescanner API to handle file I/O
+! Test `-E`: for the corresponding frontend actions the driver relies on the prescanner API to handle file I/O
 ! RUN: cat %s | flang-new -fc1 -E | FileCheck %s --check-prefix=PP-NOT-DEFINED
 ! RUN: cat %s | flang-new -fc1 -DNEW -E | FileCheck %s --check-prefix=PP-DEFINED
 
-! Test `-test-io` - for the corresponding frontend action (`InputOutputTestAction`) the driver handles the file I/O on its own
+! Test `-test-io`: for the corresponding frontend action (`InputOutputTestAction`) the driver handles the file I/O on its own
 ! the corresponding action (`PrintPreprocessedAction`)
 ! RUN: cat %s | flang-new -fc1 -test-io | FileCheck %s --check-prefix=IO --match-full-lines
 ! RUN: cat %s | flang-new -fc1 -DNEW -test-io | FileCheck %s --check-prefix=IO --match-full-lines