Some debuggers use the declaration line to avoid showing variables
before they're declared. Emit them for local variables and function
parameters.

DW_AT_decl_file would be nice too, but since its value is an index
into a table built by the linker, that's dramatically harder. In
practice, with inlining disabled it's safe to assume that all a
function's variables are declared in the same file, so this should still
be pretty useful.

Change-Id: I8105818c8940cd71bc5473ec98797cce2f3f9872
Reviewed-on: https://go-review.googlesource.com/44350Reviewed-by: David Chase <drchase@google.com>