[Clang][NFC] Use const ASTContext reference in Decl Create methods
Update Create() static factory methods, CreateDeserialized() methods, and
constructors in Decl.h to accept const ASTContext& instead of ASTContext&.
This change makes ASTContext parameters const-correct for declaration
creation and deserialization, affecting all Decl subclasses declared in
Decl.h.
Special mention:
- TranslationUnitDecl::Create() and constructor: Now accept const
ASTContext& and store it as a const reference. The getASTContext()
method uses const_cast to maintain API compatibility with code that
expects a non-const reference. This is safe since the ASTContext is
always created as non-const.
Assisted-By: Claude Sonnet 4.5