[flang] Add a warning for CDEFINED declarations that have initializers (#159456)

CDEFINED declarations are similar to "extern" declarations in C. If they
have initializers, this could lead to linker errors. clang warns about
"extern" declarations with initializers. Add similar warning to flang:
```
$ flang -c cdefined.f90 -pedantic
./cdefined.f90:3:57: warning: CDEFINED variable should not have an initializer [-Wcdefined-init]
    integer(c_int), bind(C, name='c_global', CDEFINED) :: c  = 4
                                                          ^
```

Author: eugeneepshteyn

flang/include/flang/Support/Fortran-features.h

@@ -73,7 +73,7 @@ ENUM_CLASS(UsageWarning, Portability, PointerToUndefinable,
     ZeroDoStep, UnusedForallIndex, OpenMPUsage, DataLength, IgnoredDirective,
     HomonymousSpecific, HomonymousResult, IgnoredIntrinsicFunctionType,
     PreviousScalarUse, RedeclaredInaccessibleComponent, ImplicitShared,
-    IndexVarRedefinition, IncompatibleImplicitInterfaces,
+    IndexVarRedefinition, IncompatibleImplicitInterfaces, CdefinedInit,
     VectorSubscriptFinalization, UndefinedFunctionResult, UselessIomsg,
     MismatchingDummyProcedure, SubscriptedEmptyArray, UnsignedLiteralTruncation,
     CompatibleDeclarationsFromDistinctModules,

flang/lib/Semantics/resolve-names.cpp

@@ -9118,6 +9118,9 @@ void DeclarationVisitor::NonPointerInitialization(
         if (details->init()) {
           SayWithDecl(name, *name.symbol,
               "'%s' has already been initialized"_err_en_US);
+        } else if (details->isCDefined()) {
+          context().Warn(common::UsageWarning::CdefinedInit, name.source,
+              "CDEFINED variable should not have an initializer"_warn_en_US);
         } else if (IsAllocatable(ultimate)) {
           Say(name, "Allocatable object '%s' cannot be initialized"_err_en_US);
         } else if (ultimate.owner().IsParameterizedDerivedType()) {

flang/test/Semantics/cdefined.f90

@@ -0,0 +1,6 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic -Werror
+module m
+  use iso_c_binding
+  !WARNING: CDEFINED variable should not have an initializer [-Wcdefined-init]
+  integer(c_int), bind(C, name='c_global', CDEFINED) :: c  = 42
+end