[DRAFT] Make VoidType a unit type

[ThomasHaas]

This PR gives makes VoidType a unit type with a single value unit.
This allows for a uniform treatment (and thus less code) of function calls with return value and without return values.
In fact, there are no functions without return values anymore, a void function returns the unit value. The class VoidFunctionCall becomes obsolete.
The callsite now has to assign the return value to a register of void type:

call myFunc(...);  // void functions
// ... becomes
void _ <- myFunc(...); // "_" is a register of void type.

Issues/Possible TODOs:

• The SPIRV parser does not like to reuse the same register for multiple operations, so I had to hack in an exception to this rule for the unit/void register (a reason why this is a draft).
• We should also choose a uniform name for this kind of register that cannot collide with user-provided names. For LLVM, the name _ is fine, but it might not be for SPIRV?
• We might also want to rename VoidType to UnitType, or even replace it entirely by an empty aggregate type.
• There are currently no restrictions to ensure that void types are not stored into memory, but we will likely throw exceptions in the backend if that happens.
• We might need an equality expression on VoidType if we want to use it as a proper unit type.
• [DONE] Now that VoidFunctionCall is gone, we can merge ValueFunctionCall into the base class FunctionCall.
• There are likely still code places left that can be made more uniform now.