compiler: pass a Location to import_expression

Separate the Location that import_expression uses when creating a new
Expression from the Location used to report an error.  This is a step
toward importing expressions for inlined functions.  This is a pure
refactoring that does not affect compiler behavior.

Change-Id: I731bec861c59cd70c1bc8fb0f7208f8f11766565
Reviewed-on: https://go-review.googlesource.com/c/150064
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
5 files changed