[clang] [ASTMatchers] fix `isStaticStorageClass` not matching definitions of forward declared functions (PR #120027)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

Another solution would be to explicitly document the definition is not matching with `isStaticStorageClass`, and we add `hasCanonicalDecl`. That solution would do less behind the scenes at the cost of potentially tripping poeple up (though documenting this for `isStaticStorageClass` would help).

https://github.com/llvm/llvm-project/pull/120027