[cxxmodules] Implement mapping between identifier and a library.

This patch allows given a name to deduce the library it is defined in.
It uses the fact that we have a module per library. The lookup returns
a declaration whose owning module gives a hint about the corresponding
library.

This is useful to implement the TCling::AutoLoad interface for modules.

core/metacling/src/TCling.cxx

@@ -6679,6 +6679,25 @@ const char* TCling::GetSharedLibs()
    return fSharedLibs;
 }
 
+static const char* GetClassSharedLibsForModule(const char* cls,
+                                               cling::LookupHelper &LH)
+{
+   if (!cls || !*cls)
+      return 0;
+
+   if (const clang::Decl* D = LH.findScope(cls, cling::LookupHelper::NoDiagnostics)) {
+      if (!D->isFromASTFile()) {
+         if (gDebug > 5)
+            Warning("GetClassSharedLibsForModule",
+                    "Decl found for %s is not part of a module", cls);
+         return 0;
+      }
+      clang::Module *M = D->getOwningModule()->getTopLevelModule();
+      return M->Name.c_str();
+   }
+   return 0;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Get the list of shared libraries containing the code for class cls.
 /// The first library in the list is the one containing the class, the
@@ -6687,6 +6706,10 @@ const char* TCling::GetSharedLibs()
 
 const char* TCling::GetClassSharedLibs(const char* cls)
 {
+   if (fCxxModulesEnabled)
+      if (const char* lib = GetClassSharedLibsForModule(cls, fInterpreter->getLookupHelper()))
+         return lib;
+
    if (!cls || !*cls) {
       return 0;
    }