expressions. We are now missing template instantiation logic for only
three classes of expressions:
  - Blocks-related expressions (BlockExpr, BlockDeclRefExpr)
  - C++ default argument expressions
  - Objective-C expressions

Additionally, our handling of DeclRefExpr is still quite poor, since
it cannot handle references to many kinds of declarations.

As part of this change, converted the TemplateExprInstantiator to use
iteration through all of the expressions via clang/AST/StmtNodes.def,
ensuring that we don't forget to add template instantiation logic for
any new expression node kinds.

llvm-svn: 72303