[Support] Introduce llvm::formatv() function.

This introduces a new type-safe general purpose formatting
library.  It provides compile-time type safety, does not require
a format specifier (since the type is deduced), and provides
mechanisms for extending the format capability to user defined
types, and overriding the formatting behavior for existing types.

This patch additionally adds documentation for the API to the
LLVM programmer's manual.

Mailing List Thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-October/105836.html

Differential Revision: https://reviews.llvm.org/D25587

llvm-svn: 286682