Skip declarations from external files

Summary: We don't need to traverse declarations coming from external file (c++ module). As pcm file will be added in the next diff it will make the traversal very slow. So, we're skipping these declarations in this diff

Reviewed By: phlalx

Differential Revision: D95952152

fbshipit-source-id: accbf6ae46ce938313b0150371984ba721b94c5f

Author: iamirzhan

glean/lang/clang/ast.cpp

@@ -2716,6 +2716,11 @@ struct ASTVisitor : public clang::RecursiveASTVisitor<ASTVisitor> {
   }
 
   bool TraverseDecl(clang::Decl* decl) {
+    // Skip declarations from modules/PCM files and continue traversing the AST
+    if (decl && decl->isFromASTFile()) {
+      return true;
+    }
+
     clang::DeclContext* context = nullptr;
 
     folly::Optional<Cxx::Declaration> gleanDecl;