[Support] Add the 'Error' class for structured error handling.

This patch introduces the Error classs for lightweight, structured,
recoverable error handling. It includes utilities for creating, manipulating
and handling errors. The scheme is similar to exceptions, in that errors are
described with user-defined types. Unlike exceptions however, errors are
represented as ordinary return types in the API (similar to the way
std::error_code is used).

For usage notes see the LLVM programmer's manual, and the Error.h header.
Usage examples can be found in unittests/Support/ErrorTest.cpp.

Many thanks to David Blaikie, Mehdi Amini, Kevin Enderby and others on the
llvm-dev and llvm-commits lists for lots of discussion and review.

llvm-svn: 263609