This splits out the SourcePosition class into a separate header file.
Reason for this refactoring is that said class is mostly used by the
Crankshaft compiler and not needed for all compilers. Also having the
assembler depend on the class creates a dependency cycle.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1581083009

Cr-Commit-Position: refs/heads/master@{#33325}