Fix for issue #535.

The symbols must have weak_odr linkage if they result from a template instantiation.
Small code modifications to Ir-Classes.

Adds some constructors and moves the code to the header file. Uses some of the new constructors.

A big problem with the source are the different strategies used for otherwise similar classes.
E.g. a IrField registers itself with the VarDeclaration. Same is required for IrParameter, but
in this case it is done by the caller.
[experimental] Add sanitizer options to LDC.

Add some of the sanitizer passes to LDC. This is not complete (linking must be
done using clang and the right `-fsanitize=` option) and may not be useful at
all.

If it proves to be usefull then a lot of other options (e.g. blacklist) must be
added.