[AST] Don't store data for GNU range case statement if not needed

Don't store the data for case statements of the form LHS ... RHS if not
needed. This cuts the size of CaseStmt by 1 pointer + 1 SourceLocation in
the common case.

Also use the newly available space in the bit-fields of Stmt to store the
keyword location of SwitchCase and move the small accessor
SwitchCase::getSubStmt to the header.

Differential Revision: https://reviews.llvm.org/D53609

Reviewed By: rjmccall

llvm-svn: 345472