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

This should help address a build failure found after
09117b21

../tools/clang/lib/Sema/SemaDeclCXX.cpp: In member function ‘void clang::Sema::DiagnoseStaticAssertDetails(const clang::Expr*)’:
../tools/clang/lib/Sema/SemaDeclCXX.cpp:16666:19: error: declaration of ‘const clang::Expr* clang::Sema::DiagnoseStaticAssertDetails(const clang::Expr*)::<unnamed struct>::Expr’ changes meaning of ‘Expr’ [-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: ‘Expr’ declared here as ‘class clang::Expr’
  109 | class Expr : public ValueStmt {
      |       ^~~~