[flang] Improve "Error reading module file" error message

[klausler]

Summary

Instead of using a message attachment with further details,
emit the details as part of a single message.

[PeteSteinfeld]

All builds, tests, and looks good.

[awarzynski]

This makes the diagnostics more concise, thank you!

flang/lib/Semantics/mod-file.cpp

Scope *ModFileReader::Read(const SourceName &name, Scope *ancestor) {
   CHECK(modSymbol.has<ModuleDetails>());
   CHECK(modSymbol.test(Symbol::Flag::ModFile));
   return modSymbol.scope();
 }
 
 parser::Message &ModFileReader::Say(const SourceName &name,
     const std::string &ancestor, parser::MessageFixedText &&msg,
     const std::string &arg) {
-  return context_
-      .Say(name,
-          ancestor.empty()
-              ? "Error reading module file for module '%s'"_err_en_US
-              : "Error reading module file for submodule '%s' of module '%s'"_err_en_US,
-          name, ancestor)
-      .Attach(name, std::move(msg), arg);
+  return context_.Say(name, "Cannot read module file for %s: %s"_err_en_US,
+      parser::MessageFormattedText{ancestor.empty()
+              ? "module '%s'"_en_US
+              : "submodule '%s' of module '%s'"_en_US,
+          name, ancestor}
+          .MoveString(),
+      parser::MessageFormattedText{std::move(msg), arg}.MoveString());
 }
 
 // program was read from a .mod file for a submodule; return the name of the
 // submodule's parent submodule, nullptr if none.
 static std::optional<SourceName> GetSubmoduleParent(
     const parser::Program &program) {
   CHECK(program.v.size() == 1);
   auto &unit{program.v.front()};

flang/test/Flang-Driver/include-module.f90

 ! RUN: not %flang-new -fc1 -fsyntax-only -module-dir %S/Inputs %s  2>&1 | FileCheck %s --check-prefix=SINGLEINCLUDE
 ! RUN: not %flang-new -fc1 -fsyntax-only -J %S/Inputs/module-dir -J %S/Inputs/ %s 2>&1 | FileCheck %s --check-prefix=DOUBLEINCLUDE
 ! RUN: not %flang-new -fc1 -fsyntax-only -J %S/Inputs/module-dir -module-dir %S/Inputs/ %s 2>&1 | FileCheck %s --check-prefix=DOUBLEINCLUDE
 ! RUN: not %flang-new -fc1 -fsyntax-only -module-dir %S/Inputs/module-dir -J%S/Inputs/ %s 2>&1 | FileCheck %s --check-prefix=DOUBLEINCLUDE
 
 !-----------------------------------------
 ! EXPECTED OUTPUT FOR MISSING MODULE FILE
 !-----------------------------------------
-! SINGLEINCLUDE:Error reading module file for module 'basictestmoduletwo'
-! SINGLEINCLUDE-NOT:Error reading module file for module 'basictestmoduletwo'
+! SINGLEINCLUDE:error: Cannot read module file for module 'basictestmoduletwo'
+! SINGLEINCLUDE-NOT:error: Cannot read module file for module 'basictestmoduletwo'
 ! SINGLEINCLUDE-NOT:error: Derived type 't1' not found
 ! SINGLEINCLUDE:error: Derived type 't2' not found
 
 !-----------------------------------------
 ! EXPECTED OUTPUT FOR MISSING MODULE FILE
 !-----------------------------------------
 ! DOUBLEINCLUDE:error: Only one '-module-dir/-J' option allowed
 
 !---------------------------------------
 ! EXPECTED OUTPUT FOR ALL MODULES FOUND
 !---------------------------------------
-! INCLUDED-NOT:Error reading module file
+! INCLUDED-NOT:error: Cannot read module file
 ! INCLUDED-NOT:error: Derived type 't1' not found
 ! INCLUDED:error: Derived type 't2' not found
 
 program test_dash_I_with_mod_files
     USE basictestmoduleone
     USE basictestmoduletwo
     type(t1) :: x1 ! t1 defined in Inputs/basictestmoduleone.mod
     type(t2) :: x2 ! t2 defined in Inputs/module-dir/basictestmoduleone.mod
 end

flang/test/Semantics/resolve12.f90

 ! RUN: %S/test_errors.sh %s %t %f18
 module m1
 end
 
 subroutine sub
 end
 
 use m1
-!ERROR: Error reading module file for module 'm2'
+!ERROR: Cannot read module file for module 'm2': Source file 'm2.mod' was not found
 use m2
 !ERROR: 'sub' is not a module
 use sub
 end

flang/test/Semantics/resolve26.f90

interface
     module subroutine s()
     end subroutine
   end interface
 end
 
 submodule(m1) s1
 end
 
-!ERROR: Error reading module file for submodule 's1' of module 'm2'
+!ERROR: Cannot read module file for submodule 's1' of module 'm2': Source file 'm2-s1.mod' was not found
 submodule(m2:s1) s2
 end
 
-!ERROR: Error reading module file for module 'm3'
+!ERROR: Cannot read module file for module 'm3': Source file 'm3.mod' was not found
 submodule(m3:s1) s3
 end