[C++ Patch] One more error + error to error + inform and a subtler issue

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

today I noticed the below while I was putting together another batch of 
minor diagnostic fixes. The error + error to error + inform change seems 
rather straightforward to me and as usual adds clarity to the diagnostic 
outputs (all the front-ends I have at hand either do something similar 
or do not output the inform part at all). However, there is a subtler 
issue. For example the first pair of errors we currently output for 
constexpr-specialization.C is the following (I'm removing original code 
and carets for clarity):

constexpr-specialization.C:7:26: error: redeclaration âconstexpr int 
foo(T) [with T = int]â differs in âconstexprâ

constexpr-specialization.C:6:16: error: from previous declaration 
âconstexpr int foo(T) [with T = int]â

see? The pretty printing of the previous declaration is very misleading 
because it has constexpr in it! The same happens for the dual type of 
error. Now, I suppose we should investigate that a bit more, I don't 
think it's the first time we notice this, but I'm wondering if for the 
time being we could just apply something like the below, which just does 
away with the prettyprinting for the inform, just uses "previous 
declaration here", something that in any case we already do in a number 
of places...

Thanks,
Paolo.

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