[Flang] Do not emit numeric_storage_size into object file (llvm#131463)

The value of numeric_storage_size depends on compilation options and
therefore its value is not yet known when building the builtins runtime.
Instead, the parameter is folding a __numeric_storage_size() expression
which is loaded into the user program. For the iso_fortran_env object
file, omit the symbol as it is never used.

Similar tests that ensure that __numeric_storage_size() is not folded
until compiling the actual user program exist in FortranEvalutate:

https://github.com/llvm/llvm-project/blob/1e6ba3cd2fe96be00b6ed6ba28b3d9f9271d784d/flang/lib/Evaluate/check-expression.cpp#L487-L492

https://github.com/llvm/llvm-project/blob/1e6ba3cd2fe96be00b6ed6ba28b3d9f9271d784d/flang/lib/Evaluate/fold-integer.cpp#L1457-L1460

Required for using CMake to compile the builtin module files. See RFC at
https://discourse.llvm.org/t/rfc-building-flangs-builtin-mod-files/84626

Author: Meinersbur

flang/lib/Lower/Bridge.cpp

@@ -5967,10 +5967,27 @@ class FirConverter : public Fortran::lower::AbstractConverter {
           Fortran::lower::pft::getScopeVariableListMap(mod);
       for (const auto &var : Fortran::lower::pft::getScopeVariableList(
                mod.getScope(), scopeVariableListMap)) {
+
         // Only define the variables owned by this module.
         const Fortran::semantics::Scope *owningScope = var.getOwningScope();
-        if (!owningScope || mod.getScope() == *owningScope)
-          Fortran::lower::defineModuleVariable(*this, var);
+        if (owningScope && mod.getScope() != *owningScope)
+          continue;
+
+        // Very special case: The value of numeric_storage_size depends on
+        // compilation options and therefore its value is not yet known when
+        // building the builtins runtime. Instead, the parameter is folding a
+        // __numeric_storage_size() expression which is loaded into the user
+        // program. For the iso_fortran_env object file, omit the symbol as it
+        // is never used.
+        if (var.hasSymbol()) {
+          const Fortran::semantics::Symbol &sym = var.getSymbol();
+          const Fortran::semantics::Scope &owner = sym.owner();
+          if (sym.name() == "numeric_storage_size" && owner.IsModule() &&
+              DEREF(owner.symbol()).name() == "iso_fortran_env")
+            continue;
+        }
+
+        Fortran::lower::defineModuleVariable(*this, var);
       }
       for (auto &eval : mod.evaluationList)
         genFIR(eval);