[AST] Refactor PredefinedExpr

Make the following changes to PredefinedExpr:

1. Move PredefinedExpr below StringLiteral so that it can use its definition.
2. Rename IdentType to IdentKind to be more in line with clang's conventions,
   and propagate the change to its users.
3. Move the location and the IdentKind into the newly available space of
   the bit-fields of Stmt.
4. Only store the function name when needed. When parsing all of Boost,
   of the 1357 PredefinedExpr 919 have no function name.

Differential Revision: https://reviews.llvm.org/D53605

Reviewed By: rjmccall

llvm-svn: 345460