[clang][AST] Get rid of an alignment hack in DeclObjC.h [NFCI]

This code currently uses a union object to increase the
alignment of the type ObjCTypeParamList. The original intent of this
trick was to be able to use the expression `this + 1` to access the
beginning of a tail-allocated array of `ObjCTypeParamDecl *` pointers.

The code has since been refactored and uses `llvm::TrailingObjects` to
manage the tail-allocated array. This template takes care of
alignment, so the hack is no longer necessary.

This patch removes the union so that the `SourceRange` class can be
used directly instead of being re-implemented with raw representations
of source locations.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D94224