Fix #if-skipping to *really* skip the skipped group.

Fix #if-skipping to *really* skip the skipped group.

Previously we were avoiding printing within a skipped group, but we
were still evluating directives such as #define and #undef and still
emitting diagnostics for things such as macro calls with the wrong
number of arguments.

Add a test for this and fix it with a high-priority rule in the lexer
that consumes the skipped content.