[Basic] Cleanups in IdentifierInfo following the removal of PTH

The Entry pointer in IdentifierInfo was only null for IdentifierInfo
created from a PTH. Now that PTH support has been removed we can remove
some PTH specific code in IdentifierInfo::getLength and
IdentifierInfo::getNameStart.

Also make the constructor of IdentifierInfo private to make sure that
they are only created by IdentifierTable, and move it to the header so
that it can be inlined in IdentifierTable::get and IdentifierTable::getOwn.

Differential Revision: https://reviews.llvm.org/D54866

Reviewed By: erichkeane

llvm-svn: 348384