C++ PATCH for c++/42137 (lookup of A::A in ctor-initializer)

[Jason Merrill]

After DR 147, A::A is interpreted to mean the constructor for A, but DR 
318 amended that to only apply in situations where a type name isn't 
required by the syntax.  In a ctor-initializer, if we see a 
nested-name-specifier we know that we're initializing a base class, so 
we can find the type.

This patch also improves some diagnostics for when lookup finds the 
constructor instead of the type that the user intended, and for giving 
the wrong number of template<> headers.

Tested x86_64-pc-linux-gnu, applied to trunk.
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