Declare frontend structs as extern cpp

[edi33416]

This PR is useful to the the C++ header generator to generate the dmd frontend header files that are used by gdc.

This is necessary because the generator takes into consideration only structs that are declared extern (C++)

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[thewilsonator]

Hmm,

dmd/constfold.d(98): Error: template instance `dmd.complex.complex_t.opUnary!"-"` Internal Compiler Error: C++ `string` template value parameter is not supported

[ibuclaw]

This is an internal struct? I think this can be ignored.

[edi33416]

This is because in FuncDeclaration we store a CompiledCtfeFunctionPimpl member field which was manually re-written in the declaration.h header file as CompiledCtfeFunction * (link)

[edi33416]

I also feel that this should stay private, but this was the easiest solution I came up with. Open to suggestions

[ibuclaw]

Ahh... used in func.d, in a rather weird way when comparing against the existing C++ header.

[edi33416]

buildkite failure seems to be unrelated