[AST][NFC] Add const children() accessors to all AST nodes

Systematically add the const-qualified version of children()
to all statement/expression nodes. Previously the const-qualified
variant was only defined for some nodes. NFC.

Patch by: Nicolas Manichon

Differential Revision: https://reviews.llvm.org/D60029

Reviewed By: riccibruno

llvm-svn: 358288