[AST] Pack CXXDependentScopeMemberExpr

Use the newly available space in the bit-fields of Stmt. Additionally store
FirstQualifierFoundInScope as a trailing object since it is most of the time
null (non-null for 2 of the 35446 CXXDependentScopeMemberExpr when parsing
all of Boost).

It would be possible to move the data for the nested-name-specifier to a
trailing object too to save another 2 pointers, however doing so did actually
regress the time taken to parse all of Boost slightly.

This saves 8 bytes + 1 pointer per CXXDependentScopeMemberExpr in the vast
majority of cases.

Differential Revision: https://reviews.llvm.org/D56367

Reviewed By: rjmccall

llvm-svn: 350625