Do not use the same identifier for a data member as a type; NFC

Do not use the same identifier for a data member as a type; NFC

This should help address a build failure found after
09117b21890c652994f7ada0229d309b35b44259

../tools/clang/lib/Sema/SemaDeclCXX.cpp: In member function \91void clang::Sema::DiagnoseStaticAssertDetails(const clang::Expr*)\92:
../tools/clang/lib/Sema/SemaDeclCXX.cpp:16666:19: error: declaration of \91const clang::Expr* clang::Sema::DiagnoseStaticAssertDetails(const clang::Expr*)::<unnamed struct>::Expr\92 changes meaning of \91Expr\92 [-fpermissive]
16666 |       const Expr *Expr;
      |                   ^~~~
In file included from ../tools/clang/include/clang/AST/DeclCXX.h:22,
                 from ../tools/clang/include/clang/AST/ASTLambda.h:18,
                 from ../tools/clang/lib/Sema/SemaDeclCXX.cpp:15:
../tools/clang/include/clang/AST/Expr.h:109:7: note: \91Expr\92 declared here as \91class clang::Expr\92
  109 | class Expr : public ValueStmt {
      |       ^~~~