More fixes for block mangling.

Make sure we properly treat names defined inside a block as local
names.  There are basically three fixes here.  One, correctly
treat blocks as a context where we need to use local-name mangling using
the new isLocalContainerContext helper. Two, make
CXXNameMangler::manglePrefix handle local names in a consistent way.
Three, extend CXXNameMangler::mangleLocalName so it can mangle a block
correctly.