[clang] Reapply "[clang] Introduce [[clang::lifetime_capture_by(X)]] (PR #115823)

[Utkarsh Saxena via cfe-commits]

================
@@ -3919,7 +3919,24 @@ static void HandleLifetimeCaptureByAttr(Sema &S, Decl *D,
 
 void Sema::LazyProcessLifetimeCaptureByParams(FunctionDecl *FD) {
   bool HasImplicitThisParam = isInstanceMethod(FD);
-
+  SmallVector<LifetimeCaptureByAttr *, 1> Attrs;
+  for (ParmVarDecl *PVD : FD->parameters())
+    if (auto *A = PVD->getAttr<LifetimeCaptureByAttr>())
+      Attrs.push_back(A);
+  if (HasImplicitThisParam) {
+    TypeSourceInfo *TSI = FD->getTypeSourceInfo();
+    if (!TSI)
+      return;
+    AttributedTypeLoc ATL;
+    for (TypeLoc TL = TSI->getTypeLoc();
+         (ATL = TL.getAsAdjusted<AttributedTypeLoc>());
+         TL = ATL.getModifiedLoc()) {
+      if (auto *A = ATL.getAttrAs<LifetimeCaptureByAttr>())
+        Attrs.push_back(const_cast<LifetimeCaptureByAttr *>(A));
+    }
+  }
+  if (Attrs.empty())
+    return;
----------------
usx95 wrote:

I think that is essentially what we are doing rn. We still need to check whether the parameters have attributes or not.

https://github.com/llvm/llvm-project/pull/115823