Merge #1364 #1365 #1368

1364: Allow linemap to be optional nullptr r=philberty a=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.

1365: Support extern-blocks in ast-dumps r=philberty a=philberty

This allows us to support really basic expressions and extern blocks. These
are used for the hello world version of importing metadata in crates.


1368: Fix bad ABI from string method r=philberty a=philberty

We use this method to turn the string ABI into the enum. The
rust abi was wrongly turned into the C ABI always which was
causing issues on the mangling logic for extern crate items.

Co-authored-by: Philip Herron <philip.herron@embecosm.com>