Support #include <filename>

[laurenthuberdeau]

Context

Instead of having to hide #include <filename> directives, we want to parse them but do nothing. Eventually, we'll probably want to revisit this when/if we can compile standard library files, but for now we implicitly include a runtime library, so those includes can simply be ignored.

[feeley]

This assumes that the filename has a syntax that is a valid concatenation of tokens... which will fail in some cases. But for now this is OK for the usual builtin header files.