Error messages can have a list of attachments; these are used to point
to related source locations, supply additional information, and to
encapsulate error messages that were *not* emitted in a given context
to explain why a warning was justified.
This patch adds a message severity ("Because") for that last case,
and extends to AttachTo() API to provide a means for overriding
the severity of an attached message.
Some existing message attachments had their severities adjusted,
now that we're printing them. And operator==() for Message was
cleaned up while debugging after I noticed that it was recursively
O(N**2) and subject to returning a false positive.
Differential Revision: https://reviews.llvm.org/D123710
ef141aec3c