We used to intercept function definitions, but not declarations.
GenericNamedPropertySetterCallback now also intercepts function declarations.

For definitions, we call DeclareGlobal and then InitializeVarGlobal. For
declarations, we never call InitializeVarGlobal, thus we must check for
interceptors in DeclareGlobal.

If the semantics of a redeclaration are wrong, e.g., redeclaring a read-only
property, an exception is thrown independent of whether an interceptor is
installed. Usually, i.e., not during a declaration, we only throw if
the call is not successfully intercepted.

BUG=v8:5375

Review-Url: https://codereview.chromium.org/2334733002
Cr-Commit-Position: refs/heads/master@{#39450}