Fix a performance bug with FileLocation.

The bug was actually located in SourceLocation. The constructor
accesses this.line and `this.column` in preference to the arguments it
was passed, which was fine for normal SourceLocations but for
FileLocations caused two binary searches to be performed every time a
location was instantiated.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1328613002 .