Avoid including ScopeInfo.h from Sema.h

Summary:
This provides no measurable build speedup, but it reinstates an
optimization from r112038 that was lost in r179618.  It requires moving
CapturedScopeInfo::Capture out to clang::sema, which might be too
general since we have plenty of other Capture records in BlockDecl and
other AST nodes.

Reviewers: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D44221

llvm-svn: 326957