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/lexer.lpp

@@ -251,7 +251,7 @@ string_literal        \"([^\\\"\n\v\f\r]|\\.)*\"
       // "Reading a token: ".
       llvm::errs() << "\n";
     }
-    FATAL_COMPILATION_ERROR(context.SourceLoc())
+    FATAL_COMPILATION_ERROR(context.source_loc())
         << "Invalid escaping in string: " << yytext;
   }
   return ARG_TOKEN(string_literal, *unescaped);
@@ -284,7 +284,7 @@ string_literal        \"([^\\\"\n\v\f\r]|\\.)*\"
     // "Reading a token: ".
     llvm::errs() << "\n";
   }
-  FATAL_COMPILATION_ERROR(context.SourceLoc())
+  FATAL_COMPILATION_ERROR(context.source_loc())
       << "invalid character '\\x" << llvm::toHex(llvm::StringRef(yytext, 1))
       << "' in source file.";
 }