[demangler] Avoid alignment warning

The alignment specified by a constant for the field
`BumpPointerAllocator::InitialBuffer` exceeded the alignment
guaranteed by `malloc` and `new` on Windows. This change set
the alignment value to that of `long double`, which is defined
by the used platform.

It fixes https://bugs.llvm.org/show_bug.cgi?id=37944.

Differential Revision: https://reviews.llvm.org/D48889

llvm-svn: 336312