[MLIR][SPIRV] Refactoring serialization and deserialization

This commit splits SPIR-V's serialization and deserialization code
into separate libraries. The motiviation being that the serializer
is used more often the deserializer and therefore lumping them
together unnecessarily increases binary size for the most common
case.

This commit also moves these libraries into the Target/ directory
to follow MLIR convention.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D91548