Fix: Remove IsCustomFunction bitfield to avoid size overflow

The IsCustomFunction bit pushed CXXConstructorDeclBitfields over the
64-bit limit, causing static assertion failures.

For Phase 1, we only need to parse the 'custom' keyword correctly, which
we can do without storing the flag in FunctionDecl. The parser recognizes
the keyword and DeclSpec tracks it during parsing.

Changes:
- Removed IsCustomFunction bit from FunctionDeclBitfields
- Reverted NumFunctionDeclBits back to 32
- Removed isCustomFunction()/setCustomFunction() accessor methods
- Commented out custom flag usage in SemaDecl.cpp with TODO for Phase 2

Parser and DeclSpec changes remain intact, so the keyword is still
recognized and validated. We'll add proper AST storage in Phase 2 when
implementing the transformation, potentially using an attribute or
external tracking mechanism.

Author: claude

clang/include/clang/AST/Decl.h

@@ -2904,14 +2904,6 @@ class FunctionDecl : public DeclaratorDecl,
     FunctionDeclBits.IsInline = I;
   }
 
-  /// Determine whether the "custom" keyword was specified for this function.
-  bool isCustomFunction() const { return FunctionDeclBits.IsCustomFunction; }
-
-  /// Set whether the "custom" keyword was specified for this function.
-  void setCustomFunction(bool C = true) {
-    FunctionDeclBits.IsCustomFunction = C;
-  }
-
   /// Determine whether the function was declared in source context
   /// that requires constrained FP intrinsics
   bool UsesFPIntrin() const { return FunctionDeclBits.UsesFPIntrin; }

clang/include/clang/AST/DeclBase.h

@@ -1742,8 +1742,6 @@ class DeclContext {
     uint64_t IsInline : 1;
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsInlineSpecified : 1;
-    LLVM_PREFERRED_TYPE(bool)
-    uint64_t IsCustomFunction : 1;
 
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsVirtualAsWritten : 1;
@@ -1834,7 +1832,7 @@ class DeclContext {
   };
 
   /// Number of inherited and non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = NumDeclContextBits + 33 };
+  enum { NumFunctionDeclBits = NumDeclContextBits + 32 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor

clang/lib/Sema/SemaDecl.cpp

@@ -10088,9 +10088,8 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     bool isVirtual = D.getDeclSpec().isVirtualSpecified();
     bool hasExplicit = D.getDeclSpec().hasExplicitSpecifier();
     isFriend = D.getDeclSpec().isFriendSpecified();
-    bool isCustom = D.getDeclSpec().isCustomSpecified();
-    if (isCustom)
-      NewFD->setCustomFunction(true);
+    // TODO(Phase 2): Store custom function flag when implementing transformation
+    // bool isCustom = D.getDeclSpec().isCustomSpecified();
     if (ImplicitInlineCXX20 && isFriend && D.isFunctionDefinition()) {
       // Pre-C++20 [class.friend]p5
       //   A function can be defined in a friend declaration of a