[Clang] support C23 constexpr struct member access in constant expressions (#182770)

Fixes #178349

--- 

This patch resolves an issue where accessing C23 `constexpr` struct
members using the dot operator was not recognized as a constant
expression.

According to C23 spec:

> 6.6p7:
> 
> An identifier that is:
> — an enumeration constant,
> — a predefined constant, or
> — declared with storage-class specifier constexpr and has an object
type,
> is a named constant, as is a postfix expression that applies the .
member access operator to a named
> constant of structure or union type, even recursively. For enumeration
and predefined constants,
> their value and type are defined in the respective clauses; for
constexpr objects, such a named
> constant is a constant expression with the type and value of the
declared object.
> 
> § 6.6p13:
> 
> A structure or union constant is a named constant or compound literal
constant with structure or
> union type, respectively
> 
> § 6.6p15:
> 
> Starting from a structure or union constant, the member-access .
operator may be used to form a
> named constant or compound literal constant as described previously in
this subclause