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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189952 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "Add r159136 back now that pr13124 has been fixed."

This reverts commit r189886.

I found a corner case where this optimization is not valid:

Say we have a "linkonce_odr unnamed_addr" in two translation units:
* In TU 1 this optimization kicks in and makes it hidden.
* In TU 2 it gets const merged with a constant that is *not* unnamed_addr,
  resulting in a non unnamed_addr constant with default visibility.
* The static linker rules for combining visibility them produce a hidden
  symbol, which is incorrect from the point of view of the non unnamed_addr
  constant.

The one place we can do this is when we know that the symbol is not used from
another TU in the same shared object, i.e., during LTO. I will move it there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189954 91177308-0d34-0410-b5e6-96231b3b80d8