Allow storage classes other than static

[ForNeVeR]

C standard (6.7.1 Storage-class specifiers) defines the following storage classes:

• typedef (not a real storage class)
• extern
• static
• _Thread_local
• auto
• register

Only one storage class per declaration is allowed (with an exception of _Thread_local with either static or extern).

We should allow these storage classes in the Cesium.CodeGen.Ir.Declarations.ScopedIdentifierDeclaration.

When implementing this, find the number 343 in the sources.

This issue doesn't include full processing of these storage classes, but only allowing them in the IR classes and proper parsing.