[Basic] Fix up DiagnosticBuilder::{FlushCounts,Emit} to be inline.

 - This is much more important than it appears at first glance...

The intended design of DiagnosticBuilder was that it never escape and that all
its members would get lowered to registers by the compiler. By fixing Emit here,
the compiler can completely eliminate the DiagnosticBuilder object and never
need to push those registers back into it.

Unfortunately, Sema has broken DiagnosticBuilder in other ways (by introducing
SemaDiagnosticBuilder), so we don't get the fill impact of this, but it is still
good for 30k reduction in code size. I'll work on fixing the
SemaDiagnosticBuilder problems next.

llvm-svn: 152669