[Support] Add a free toString function for Error

toString() consumes an Error and returns a string representation of its
contents. This commit also adds a message() method to ErrorInfoBase for
convenience.

Differential Revision: http://reviews.llvm.org/D19883

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268465 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed