Insert a space at the start of a line comment in case it starts with an alphanumeric character.

Summary:
"//Test" becomes "// Test". This change is aimed to improve code
readability and conformance to certain coding styles. If a comment starts with a
non-alphanumeric character, the space isn't added, e.g. "//-*-c++-*-" stays
unchanged.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D949