[C++ PATCH] Deprecate -ffriend-injection

[Nathan Sidwell]

This patch deprecates the -ffriend-injection option.
1) issue a deprecated warning, if the option is set.
2) if we do inject a visible friend, issue a warning.  This warning is 
not inhibitable, like the for-scope behaviour.  (Notice we never 
injected visible friend classes, only functions).
3) Update the 'backwards compatibility' section to mention this.

I noticed that at the point we warn about the flag, input_location is 
BUILTINS_LOCATION.  There isn't a known fixed location for 
<command-line> or <main-input-file>.  I think using UNKNOWN_LOCATION, 
which generates 'cc1plus: diag...', is less confusing than '<builtins>: 
diag...'.

'Now that there is a definitive ISO standard C++, G++ has a 
specification to adhere to.'

We now have several :)

nathan
-- 
Nathan Sidwell
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