[flang] Fix crash with USE of hermetic module file

When one hermetic module file uses another, a later compilation
may crash in semantics when it itself is used, since the module
file reader sets the "current hermetic module file scope" to
null after reading one rather than saving and restoring that
pointer.

Author: klausler

flang/lib/Semantics/mod-file.cpp

@@ -1537,6 +1537,7 @@ Scope *ModFileReader::Read(SourceName name, std::optional<bool> isIntrinsic,
   // created under -fhermetic-module-files?  If so, process them first in
   // their own nested scope that will be visible only to USE statements
   // within the module file.
+  Scope *previousHermetic{context_.currentHermeticModuleFileScope()};
   if (parseTree.v.size() > 1) {
     parser::Program hermeticModules{std::move(parseTree.v)};
     parseTree.v.emplace_back(std::move(hermeticModules.v.front()));
@@ -1552,7 +1553,7 @@ Scope *ModFileReader::Read(SourceName name, std::optional<bool> isIntrinsic,
   GetModuleDependences(context_.moduleDependences(), sourceFile->content());
   ResolveNames(context_, parseTree, topScope);
   context_.foldingContext().set_moduleFileName(wasModuleFileName);
-  context_.set_currentHermeticModuleFileScope(nullptr);
+  context_.set_currentHermeticModuleFileScope(previousHermetic);
   if (!moduleSymbol) {
     // Submodule symbols' storage are owned by their parents' scopes,
     // but their names are not in their parents' dictionaries -- we

flang/test/Semantics/modfile75.F90

@@ -0,0 +1,17 @@
+!RUN: %flang -c -fhermetic-module-files -DWHICH=1 %s && %flang -c -fhermetic-module-files -DWHICH=2 %s && %flang_fc1 -fdebug-unparse %s | FileCheck %s
+
+#if WHICH == 1
+module modfile75a
+  use iso_c_binding
+end
+#elif WHICH == 2
+module modfile75b
+  use modfile75a
+end
+#else
+program test
+  use modfile75b
+!CHECK: INTEGER(KIND=4_4) n
+  integer(c_int) n
+end
+#endif