[libc++] Clean up logic around aligned/sized allocation and deallocation

Due to the need to support compilers that implement builtin operator
new/delete but not their align_val_t overloaded versions, there was a
lot of complexity. By assuming that a compiler that supports the builtin
new/delete operators also supports their align_val_t overloads, the code
can be simplified quite a bit.

Differential Revision: https://reviews.llvm.org/D88301