Reapply r207876 (Try simplifying LexicalScopes ownership again) including a...

Reapply r207876 (Try simplifying LexicalScopes ownership again) including a workaround for an MSVC2012 bug regarding forward_as_tuple

(r207876 was reverted in r208131 after seeing some consistent buildbot
failure for MSVC 2012. The original commits were in r207724-r207726)

Takumi was nice enough to dig into this and locate this Microsoft
Connect issue:
http://connect.microsoft.com/VisualStudio/feedback/details/814899/forward-as-tuple-debug-implementation-error
describing a bug in MSVC2012's forward_as_tuple implementation.

Since the parameters in this instance are trivial/small, pass them by
value (using make_tuple) instead of perfectly-forwarded tuple of rvalue
references (involving the broken forward_as_tuple). Hopefully this will
satisfy MSVC2012.

llvm-svn: 208364