Initial work on refactoring GNU runtime code (long overdue - it's quite obvious

that I hadn't used C++ for several years before writing most of this code).
Still lots more to do.  This set of changes includes:

- Remove the distinction between typed and untyped selectors.  More accurately
  reflect what the runtime does, by using typed selectors everywhere, with an
  empty type field if the types are unknown.  Now we just store a small list of
  types for each selector (in theory, this should always be exactly one, but
  this constraint was not enforced back in 1986 when it should have been).

- Add some consistency to how runtime functions are created.  These are all
  generated via the LazyRuntimeFunction class (which might be useful outside
  CGObjCGNU - feel free to move it into a header if it is).  This function
  stores the types of a function, looks it up the first time it's used, and
  caches the result.  This means that we're now not wasting time constructing
  the llvm::FunctionType every time some of the functions are looked up, but
  also not inserting references to runtime functions into the module if they're
  not actually used.

- Started separating out the fragile and non-fragile ABI behaviours into two
  subclasses of CGObjCGNU: CGObjCGCC for the legacy GCC runtime ABI and
  CGObjCGNUstep for the new GNUstep ABI.  Not all of the differences in
  behaviour are factored out yet, but they will be in future commits.

- Removed all of the CodeGen:: things: we've been using namespace CodeGen in
  this file for ages, so having explicit namespace specifiers is just a bit
  confusing.

- Added a few more comments.

- Used llvm::StringRef instead of std::string in a few places.

- Finally got around to storing the module path in the module structure.  The
  ABI says that the compiler should do this, although it's not used in the
  runtime or exposed outside the runtime, so it's pretty useless.  

Still to do:

- We currently have two code paths for generating try blocks, one for ObjC and
  one for ObjC++.  Not only are these substantially similar, they are also very
  similar to the CGObjCMac version.  These need factoring out into a single
  parameterised implementation, either in CGObjCRuntime or CodeGenFunction.
  The EmitObjCXXTryStmt() function was added so that the changes to fix a bug
  in time for the 2.9 release would be self-contained and reduce the chances of
  breaking anything else, but these should be done properly as soon as
  possible.

- Split up some large functions (e.g. GenerateClass()) into smaller functions
  for generating the various data structures.

- The method lookup code into the two subclasses, removing the conditionals in
  the message send functions.

- Add doxygen comments on the remaining undocumented functions.

- We seem to be generating global pointer variables for selectors, then storing
  a pointer to the selector, then generating a load of this pointer (and then a
  load of the real selector later) every time a static selector is used.  I can
  only assume I was asleep or drunk when I did this - we should just be
  referencing the selectors directly in the selector array.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@128152 91177308-0d34-0410-b5e6-96231b3b80d8

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