Move generic isPrint and columnWidth implementations to a separate...

Move generic isPrint and columnWidth implementations to a separate header/source to allow using both generic and system-dependent versions on win32.

Summary:
This is needed so we can use generic columnWidthUTF8 in clang-format on
win32 simultaneously with a separate system-dependent implementations of
isPrint/columnWidth in TextDiagnostic.cpp to avoid attempts to print Unicode
characters using narrow-character interfaces (which is not supported on Windows,
and we'll have to figure out how to handle this).

Reviewers: jordan_rose

Reviewed By: jordan_rose

CC: llvm-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D1559

llvm-svn: 189952