I'm proposing two changes:

First, rename AssertVariableIsOfType to StaticAssertVariableIsOfType. The
current name suggests that it is a run-time assertion (like "Assert"), but
it's not. The name change makes that clearer.

I doubt that the current name is used in many extensions, but if necessary,
extension code could adapt to this quite easily with something like

#if PGVERSIONNUM < ...
#define StaticAssertVariableIsOfType(x, y) AssertVariableIsOfType(x, y)
#endif

Second, change the underlying implementation of StaticAssertVariableIsOfType
to use StaticAssertDecl instead of StaticAssertStmt. This makes
StaticAssertVariableIsOfType behave more like a normal static assertion, and
in many cases we can move the current instances to a more natural position
at file scope. This is similar to previous commits like 493eb0da31b.