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/interpreter/interpreter.h

@@ -42,7 +42,7 @@ class Interpreter {
   // Attempts to match `v` against the pattern `p`. If matching succeeds,
   // returns the bindings of pattern variables to their matched values.
   auto PatternMatch(Nonnull<const Value*> p, Nonnull<const Value*> v,
-                    SourceLocation loc) -> std::optional<Env>;
+                    SourceLocation source_loc) -> std::optional<Env>;
 
   // Support TypeChecker allocating values on the heap.
   auto AllocateValue(Nonnull<const Value*> v) -> Address {
@@ -102,7 +102,7 @@ class Interpreter {
   struct CallFunction {
     Nonnull<const FunctionValue*> function;
     Nonnull<const Value*> args;
-    SourceLocation loc;
+    SourceLocation source_loc;
   };
 
   // Transition type which does nothing.
@@ -131,7 +131,8 @@ class Interpreter {
 
   void InitGlobals(const std::vector<Nonnull<const Declaration*>>& fs);
   auto CurrentEnv() -> Env;
-  auto GetFromEnv(SourceLocation loc, const std::string& name) -> Address;
+  auto GetFromEnv(SourceLocation source_loc, const std::string& name)
+      -> Address;
 
   void DeallocateScope(Nonnull<Scope*> scope);
   void DeallocateLocals(Nonnull<Frame*> frame);
@@ -143,10 +144,10 @@ class Interpreter {
       -> Nonnull<const Value*>;
 
   auto EvalPrim(Operator op, const std::vector<Nonnull<const Value*>>& args,
-                SourceLocation loc) -> Nonnull<const Value*>;
+                SourceLocation source_loc) -> Nonnull<const Value*>;
 
   void PatternAssignment(Nonnull<const Value*> pat, Nonnull<const Value*> val,
-                         SourceLocation loc);
+                         SourceLocation source_loc);
 
   void PrintState(llvm::raw_ostream& out);