move the Diag method for Sema to be inline. This shrinks the release-asserts

clang executable (when built with gcc 4.2 on the mac) from 14519740 to 
14495028 bytes.  This shrinks individual object files as well: SemaChecking
from 23580->22248, SemaDeclObjc from 61368->57376, SemaExpr from 
115628->110516, as well as several others.

llvm-svn: 59867