Issue 8765 - Assert should show failing expression. + Issue 9255.

https://github.com/D-Programming-Language/dmd/pull/1203 had to be closed because Github has some bugs with rebasing. Unlike the previous pull this one has unittests.

Fixes [Issue 8765](http://d.puremagic.com/issues/show_bug.cgi?id=8765) - Assert should print failing expression.
Fixes [Issue 9255](http://d.puremagic.com/issues/show_bug.cgi?id=9255) - Assert should print file name rather than module name, to be consistent with other Exception types.

Note that fixing 9255 is automatic after Fixing 8765 due to the change in code paths (a message now always exists and therefore the if statement is always taken).