[clang] [NFCI][clang][analyzer] Make ProgramStatePartialTrait a template definition (PR #98150)

[Balazs Benics via cfe-commits]

steakhal wrote:

> N4860 13 [class.derived]/2 mandates that base classes must be complete types. Before this patch, ProgramStatePartialTrait is a forward declaration of a class template, thus an incomplete type. Explicit specializations of forward declared templates are also incomplete types, until the body of the specialization is seen, thus should not appear as a base class. This patch makes ProgramStatePartialTrait a definition with an empty body by default, enabling it to appear in a base-specifier list, and practically eliminating a compiler warning given by GCC13.

Before we would proceed, could you please paste the diagnostic that this PR fixes?

https://github.com/llvm/llvm-project/pull/98150