Make MCSymbol::Name be a union of uint64_t and a pointer.

Make MCSymbol::Name be a union of uint64_t and a pointer.

This should hopefully fix the 32-bit bots which were allocating space for a pointer
but needed to be aligned to 64-bits.

Now we allocate enough space for a uint64_t and a pointer and cast to the appropriate storage

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239428 91177308-0d34-0410-b5e6-96231b3b80d8