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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@358288 91177308-0d34-0410-b5e6-96231b3b80d8