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/syntax/parse_and_lex_context.h

@@ -23,7 +23,7 @@ class ParseAndLexContext {
   // Writes a syntax error diagnostic containing message to standard error.
   auto PrintDiagnostic(const std::string& message) -> void;
 
-  auto SourceLoc() -> SourceLocation {
+  auto source_loc() -> SourceLocation {
     return SourceLocation(input_file_name,
                           static_cast<int>(current_token_position.begin.line));
   }