[Feat] Allow Finding across only parts of an AST.

This is relevant for clang modules, as they are imported into the AST, but are actually part of a different TU.
It can result in hundreds of milliseconds of additional time to also traverse the AST of these modules, and often for no benefit, as they are frequently already traversed in their own TU.

Author: matts1

clang/include/clang/ASTMatchers/ASTMatchFinder.h

@@ -139,6 +139,13 @@ class MatchFinder {
     ///
     /// It prints a report after match.
     std::optional<Profiling> CheckProfiling;
+
+    /// Whether to traverse a Decl. This is relevant for clang modules, as they
+    /// are imported into the AST, but are actually part of a different TU.
+    /// It can result in hundreds of milliseconds of additional time to also
+    /// traverse the AST of these modules, and often for no benefit, as they
+    /// are frequently already traversed in their own TU.
+    std::optional<llvm::function_ref<bool(const Decl *)>> ShouldTraverseDecl;
   };
 
   MatchFinder(MatchFinderOptions Options = MatchFinderOptions());

clang/lib/ASTMatchers/ASTMatchFinder.cpp

@@ -1443,7 +1443,7 @@ bool MatchASTVisitor::objcClassIsDerivedFrom(
 }
 
 bool MatchASTVisitor::TraverseDecl(Decl *DeclNode) {
-  if (!DeclNode) {
+  if (!DeclNode || (Options.ShouldTraverseDecl && !(*Options.ShouldTraverseDecl)(DeclNode))) {
     return true;
   }