[AST] Only store the needed data in IfStmt

Only store the needed data in IfStmt. This cuts the size of IfStmt
by up to 3 pointers + 1 SourceLocation. The order of the children
is intentionally kept the same even though it would be more
convenient to put the optional trailing objects last. Additionally
use the newly available space in the bit-fields of Stmt to store
the location of the "if".

The result of this is that for the common case of an
if statement of the form:

if (some_cond)
  some_statement

the size of IfStmt is brought down to 8 bytes + 2 pointers,
instead of 8 bytes + 5 pointers + 2 SourceLocation.

Differential Revision: https://reviews.llvm.org/D53607

Reviewed By: rjmccall

llvm-svn: 345464