[clang] Use range-based for to iterate over fields in record layout, NFC (PR #122029)

[Boaz Brickner via cfe-commits]

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@@ -3115,7 +3115,19 @@ class FieldDecl : public DeclaratorDecl, public Mergeable<FieldDecl> {
 
   /// Returns the index of this field within its record,
   /// as appropriate for passing to ASTRecordLayout::getFieldOffset.
-  unsigned getFieldIndex() const;
+  unsigned getFieldIndex() const {
+    const FieldDecl *C = getCanonicalDecl();
+    if (C->CachedFieldIndex == 0)
+      C->setCachedFieldIndex();
+    assert(C->CachedFieldIndex);
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bricknerb wrote:

This should be insider the if above.

https://github.com/llvm/llvm-project/pull/122029