Fix mangling for block literals.

Blocks, like lambdas, can be written in contexts which are required to be
treated as the same under ODR.  Unlike lambdas, it isn't possible to actually
take the address of a block, so the mangling of the block itself doesn't
matter. However, objects like static variables inside a block do need to
be mangled in a consistent way.

There are basically three components here. One, block literals need a
consistent numbering.  Two, objects/types inside a block literal need
to be mangled using it.  Three, objects/types inside a block literal need
to have their linkage computed correctly.