PCH optimization for the identifier table, where we separate

"interesting" identifiers (e.g., those where the IdentifierInfo has
some useful information) from "uninteresting" identifiers (where the
IdentifierInfo is just a name). This makes the hash table smaller (so
searching in it should be faster) and, when loading "uninteresting"
identifiers, we skip the lookup in the hash table.

PCH file size is slightly smaller than before (since we don't emit the
contents of the uninteresting IdentifierInfo structures). The
Cocoa.h-prefixed "Hello, World" doesn't show any speedup, although
we're getting to the point where system noise is a bit issue.

llvm-svn: 70075