Add a descriptive name for a constant. Also I'm at least temporarily waging...

Add a descriptive name for a constant.  Also I'm at least temporarily waging war on throw specs, both old and new style.  Except where we have already publicly exposed the throw spec, I'm getting rid of them.  They may come back later.  But they seem somewhat prone to cyclic dependencies here.  The throw spec implies compiler generated code that this library has to jump to during stack unwinding.  I'd like to minimize the possiblity that the code used to properly make that jump is itself creating such jumps.

llvm-svn: 149251