In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

This patch also removes redundant "const" from the in-class definitions.

Identified with readability-redundant-declaration.

- Put the element size field in the same place for all non-pointer
  types.
- Put the element size and address space fields in the same place for
  all pointer types.
- Put the number of elements and scalable fields in the same place for
  all vector types.

This simplifies initialization and accessor methods isScalable,
getElementCount, getScalarSizeInBits and getAddressSpace.

Finally addresses https://reviews.llvm.org/D148769#4311232 :)

No behavior change.