SIL: simplify the SILNode inline bitfields.

The use of the SWIFT_INLINE_BITFIELD macros in SILNode were a constant source of confusion and bugs.
With this refactoring I tried to simplify the definition of "shared fields" in SILNode, SILValue and SILInstruction classes:

* Move `kind`, `locationKindAndFlags` and the 32-bit fields out of the 64-bitfield into their own member variables. This avoids _a lot_ of manual bit position computations.
* Now we have two separate "shared fields": an 8-bit field (e.g. for boolean flags) and a 32-bit field (e.g. for indices, which can potentially get large). Both fields can be used independently. Also, they are not "bit fields" per se. Instructions can use the field e.g. as a `bool`, `uint32_t`, or  - if multiple flags are to be stored - as a packed bit field.
* With these two separate fields, we don't have the need for defining bitfields both in a base class _and_ in a derived value/instruction class. We can get rid of the complex logic which handles such cases. Just keep a check to catch accidental overlaps of fields in base and derived classes.
* Still use preprocessor macros for the implementation, but much simpler ones than before.
* Add documentation.

Author: eeckstein

include/swift/SIL/SILArgument.h

@@ -68,6 +68,7 @@ class SILArgument : public ValueBase {
 
   SILBasicBlock *parentBlock;
   const ValueDecl *decl;
+  USE_SHARED_UINT8;
 
 protected:
   SILArgument(ValueKind subClassKind, SILBasicBlock *inputParentBlock,
@@ -81,19 +82,19 @@ class SILArgument : public ValueBase {
                        const ValueDecl *inputDecl = nullptr)
       : ValueBase(subClassKind, type),
         parentBlock(nullptr), decl(inputDecl) {
-    Bits.SILArgument.VOKind = static_cast<unsigned>(ownershipKind);
+    sharedUInt8().SILArgument.valueOwnershipKind = uint8_t(ownershipKind);
   }
 
 public:
   void operator=(const SILArgument &) = delete;
   void operator delete(void *, size_t) = delete;
 
   ValueOwnershipKind getOwnershipKind() const {
-    return static_cast<ValueOwnershipKind>(Bits.SILArgument.VOKind);
+    return ValueOwnershipKind(sharedUInt8().SILArgument.valueOwnershipKind);
   }
 
   void setOwnershipKind(ValueOwnershipKind newKind) {
-    Bits.SILArgument.VOKind = static_cast<unsigned>(newKind);
+    sharedUInt8().SILArgument.valueOwnershipKind = uint8_t(newKind);
   }
 
   SILBasicBlock *getParent() const { return parentBlock; }