Need representation for explicit(expr)

[GorNishanov]

In IPR, we have

                                // -- DeclSpecifiers --
   enum class DeclSpecifiers : uint32_t {
      None       = 0,
      ...
      Inline     = 1 << 5,
      Virtual    = 1 << 6,   // also used as storage class specifier
                             // for virtual base subobjects
      Explicit   = 1 << 7,
      Pure       = 1 << 8,
      FunctionSpecifier = Inline | Virtual | Explicit | Pure,
      ...

We need to add support for C++20 explicit(expr).