Implemented llvm::sys::locale::columnWidth and isPrint for the case of generic UTF8-capable terminal.

Summary:
This is a second attempt to get this right. After reading the Unicode
Standard I came up with the code that uses definitions of "printable" and
"column width" more suitable for terminal output (i.e. fixed-width fonts and
special treatment of many control characters).
The implementation here can probably be used for Windows and MacOS if someone
can test it properly.
The patch addresses PR14910.

Reviewers: jordan_rose, gribozavr

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1253