Mass rename SourceLoc and Tag (#860)

This does a mass rename of:

-   `SourceLoc()` -> `source_loc()` for property naming
    - `loc` -> `source_loc_` for underscore+consistency
    - Generally changing function args to `source_loc` for consistency
-   `Tag()` -> `kind()` for property naming and `Kind` parity
    - `tag` -> `kind_` for underscore

Also renames `Pos` and `Results` on `Action`. These are a bit of an exception in that most base classes only have `Tag` and maybe `SourceLoc`, whereas `Action` has a little more. I felt okay having `source_loc()` and `kind()` on the base class where children do `Exp()` and the like, but it felt weird to me to mix it on the same class.

The reason for doing this cross-class in one PR is so that I can do it efficiently with a global replace in the codebase, rather than e.g. changing `Expression` but having to read through compiler errors to determine where it's calling `Expression`'s `Tag` versus a different `Tag`. The end result should be equivalent.

executable_semantics/ast/declaration.h

@@ -41,6 +41,7 @@ class Declaration {
   Declaration& operator=(const Member&) = delete;
 
   void Print(llvm::raw_ostream& out) const;
+  LLVM_DUMP_METHOD void Dump() const { Print(llvm::errs()); }
 
   // Returns the enumerator corresponding to the most-derived type of this
   // object.
@@ -50,7 +51,7 @@ class Declaration {
 
  protected:
   // Constructs a Declaration representing syntax at the given line number.
-  // `tag` must be the enumerator corresponding to the most-derived type being
+  // `kind` must be the enumerator corresponding to the most-derived type being
   // constructed.
   Declaration(Kind kind, SourceLocation source_loc)
       : kind_(kind), source_loc_(source_loc) {}