[AST] Use APIntStorage to fix memory leak in EnumConstantDecl. (#78311)

EnumConstantDecl is allocated by the ASTContext allocator so the
destructor is never called.

This patch takes a similar approach to IntegerLiteral by using
APIntStorage to allocate large APSInts using the ASTContext allocator as
well.

The downside is that an additional heap allocation and copy of the data
needs to be made when calling getInitValue if the APSInt is large.

Fixes #78160.