[flang] Distinguish intrinsic module USE in module files; correct search paths

[klausler]

Summary

In the USE statements that f18 emits to module files, ensure that symbols
from intrinsic modules are marked as such on their USE statements. And
ensure that the current working directory (".") cannot override the intrinsic
module search path when trying to locate an intrinsic module.

flang/lib/Parser/parsing.cpp

   const SourceFile *sourceFile;
   if (path == "-") {
     sourceFile = allSources.ReadStandardInput(fileError);
+  } else if (options.isModuleFile) {
+    // Don't mess with intrinsic module search path
+    sourceFile = allSources.Open(path, fileError);
   } else {
-    std::optional<std::string> currentDirectory{"."};
-    sourceFile = allSources.Open(path, fileError, std::move(currentDirectory));
+    sourceFile =
+        allSources.Open(path, fileError, "."s /*prepend to search path*/);
   }
   if (!fileError.str().empty()) {
     ProvenanceRange range{allSources.AddCompilerInsertion(path)};

flang/lib/Semantics/mod-file.cpp

 void ModFileWriter::PutUse(const Symbol &symbol) {
   auto &details{symbol.get<UseDetails>()};
   auto &use{details.symbol()};
-  uses_ << "use " << GetUsedModule(details).name();
-  PutGenericName(uses_ << ",only:", symbol);
+  const Symbol &module{GetUsedModule(details)};
+  if (use.owner().parent().IsIntrinsicModules()) {
+    uses_ << "use,intrinsic::";
+  } else {
+    uses_ << "use ";
+  }
+  uses_ << module.name() << ",only:";
+  PutGenericName(uses_, symbol);
   // Can have intrinsic op with different local-name and use-name
   // (e.g. `operator(<)` and `operator(.lt.)`) but rename is not allowed
   if (!IsIntrinsicOp(symbol) && use.name() != symbol.name()) {

       std::remove(options.searchDirectories.begin(),
           options.searchDirectories.end(), dir);
     }
+    options.searchDirectories.insert(options.searchDirectories.begin(), "."s);
   }
   if (isIntrinsic.value_or(true)) {
     for (const auto &dir : context_.intrinsicModuleDirectories()) {

flang/test/Semantics/modfile36.f90

 !module m1
 ! interface
 !  subroutine s(x)
-!   use iso_c_binding, only: c_ptr
+!   use,intrinsic::iso_c_binding, only: c_ptr
 !   type(c_ptr) :: x
 !  end
 ! end interface

 end module
 !Expect: m2.mod
 !module m2
-! use iso_c_binding,only:c_ptr
+! use,intrinsic::iso_c_binding,only:c_ptr
 ! interface
 !  subroutine s(x)
 !   import::c_ptr