[AST] Widen the bit-fields of Stmt to 8 bytes.

Although some classes are using the tail padding of Stmt, most of
them are not. In particular the expression classes are not using it
since there is Expr in between, and Expr contains a single pointer.

This patch widen the bit-fields to Stmt to 8 bytes and move some
data from NullStmt, CompoundStmt, LabelStmt, AttributedStmt, SwitchStmt,
WhileStmt, DoStmt, ForStmt, GotoStmt, ContinueStmt, BreakStmt
and ReturnStmt to the newly available space.

In itself this patch do not achieve much but I plan to go through each of
the classes in the statement/expression hierarchy and use this newly
available space. A quick estimation gives me that this should shrink the
size of the statement/expression hierarchy by >10% when parsing all of Boost.

Differential Revision: https://reviews.llvm.org/D53604

Reviewed By: rjmccall

llvm-svn: 345459