Add macros for diagnostic control, use them in locale/weightwc.h

[Paul Eggert]

Joseph Myers wrote:
> On Tue, 25 Nov 2014, Paul Eggert wrote:
>
>> >Joseph Myers wrote:
>>> > >It's only in a particular bit of code where we want to assume it's
>>> > >initialized.
>> >
>> >We could use ASSUME_INITIALIZED only in the particular bit of code where it's
>> >needed, as it doesn't have to be used immediately after a declaration.
> But won't the proposed asm, with its output operand for resplen, imply
> that any previous value in resplen is dead

Ah, true.  Still, if we use it just after resplen's declaration, we've localized 
the assumption well -- better than we would have done with the pragmas, and 
that's good enough.