Add Status -- llvm::Error glue

Summary:
This adds functions to convert between llvm::Error and Status classes.
Posix errors in Status are represented as llvm::ECError, and the rest as
llvm::StringError.

For the conversion from Error to Status, ECError is again represented as
a posix error in Status, while other errors are stored as generic errors
and only the string value is preserved.

Reviewers: zturner, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D33241

llvm-svn: 303348