lint: fix memory corruption in statement expressions (since 2021-12-17)

The commit that introduced the assertion failure looks innocent, it only
adds a few predefined functions for GCC mode.  Nevertheless, before that
commit, lint consistently complained about 'error: void type illegal in
expression [109]', which doesn't make sense either.

This fix also removes the creative use of the initialization stack to
store the type of the statement expression.  Having a separate stack for
these statement expressions makes the code easier to understand.

(rillig)