[flang] Support for PowerPC vector type

The following PowerPC vector type syntax is added:

  VECTOR ( element-type-spec )

where element-type-sec is integer-type-spec, real-type-sec or unsigned-type-spec.

Two opaque types (__VECTOR_PAIR and __VECTOR_QUAD) are also added.

A finite set of functionalities are implemented in order to support the new types:
1. declare objects
2. declare function result
3. declare type dummy arguments
4. intrinsic assignment between the new type objects (e.g. v1=v2)
5. reference functions that return the new types

Submit on behalf of @tislam @danielcchen

Authors: @tislam @danielcchen

Differential Revision: https://reviews.llvm.org/D150876