Flag that friend function definitions are "late parsed" so that

template instantiation will still consider them to be definitions
if we instantiate the containing class before we get around
to parsing the friend.

This seems like a legitimate use of "late template parsed" to me,
but I'd appreciate it if someone responsible for the MS feature
would look over this.

This file already appears to access AST nodes directly, which
is arguably not kosher in the parser, but the performance of this
path matters enough that perpetuating the sin is justifiable.
Probably we ought to reconsider this policy for very simple
manipulations like this.

The reason this entire thing is necessary is that
function template instantiation plays some very gross games
in order to not associate an instantiated function template
with the class it came from unless it's a definition, and
the reason *that's* necessary is that the AST currently
cannot represent the instantiation history of individual
function template declarations, but instead tracks it in
common for the entire function template.  That probably
prevents us from correctly reporting ill-formed calls to
ambiguously instantiated friend function templates.

rdar://12350696

llvm-svn: 177003