Sema: Propagate the mangling number into instantiations

We would lose track of the mangling number assigned to the original
declaration which would cause us to create manglings that didn't match
the Itanium C++ specification.

e.g. Two static fields with the same name inside of a function template
would receive the same mangling with LLVM fixing up the second field so
they wouldn't collide.  This would create an incompatibility with other
compilers following the Itanium ABI.

I've confirmed that the new mangling is identical to the ones generated
by icc and gcc.

N.B. This was uncovered while working on Microsoft mangler.