d: Set doing_semantic_analysis_p before calling functionSemantic3

As functions which have semantic analysis ran late may still require
the use of CTFE built-ins to be evaluated.

gcc/d/ChangeLog:

	* decl.cc (DeclVisitor::visit (FuncDeclaration *)): Set
	doing_semantic_analysis_p before calling functionSemantic3.

gcc/testsuite/ChangeLog:

	* gdc.dg/ctfeintrinsics.d: New test.