[C++ Patch] Tidy duplicate_decls locations

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

Hi,

these bits started when I realized that in the error messages about 
redefined default arguments we were using DECL_SOURCE_LOCATION for the 
olddecl and just input_location for newdecl: in the future we'll able to 
point at the specific default argument but for the time being we can as 
well be consistent and point at the function name for the newdecl too 
instead of the meaningless closing parenthesis. Then I realized that in 
the rather large duplicate_decls we were using a mix of 
DECL_SOURCE_LOCATION (olddecl) and newdecl - lots - and '+' in a rather 
inconsistent way.

Tested x86_64-linux.

Thanks, Paolo.

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