Allow linemap to be optional nullptr

[philberty]

The line map within the lexer is used to notify GCC of location data.
This lexer is used on raw string buffers where the linemap can be
null and thus can hit nullptr's. This patch wraps the linemap usage
behind a function. We might make the linemap mandatory as a reference
but lets figure out how the location info should look for imports first.

[CohenArthur]

LGTM

[philberty]

bors r+

[bors]

Build succeeded:

• build-and-check