Add braces for CARBON_KIND uses that lack them (#5882)

Also document why these are expected on `CARBON_KIND`. In
`kind_switch_test.cpp`, drop the `str` variable.

My recollection of the original discussion of `CARBON_KIND` is that it
should always have braces due to the risk of confusion for statement
interpretation, similar to a typical `if`/`else` but more subtle due to
the macro.

For example:

```
      case CARBON_KIND(int n):
        str << "int = " << n;
        return str.TakeStr();
```

is equivalent to:

```
      case CARBON_KIND(int n): {
          str << "int = " << n;
        }
        return str.TakeStr();
```

This happens to work in context because `str` isn't scoped, but a
trivial refactoring to move `RawStringOstream str;` the first statement
of the `case` would probably have non-obvious results. For example:

```
      case CARBON_KIND(int n):
        RawStringOstream str; // Valid name shadowing, destructed without use.
        str << "int = " << n; // Name lookup error on `n`.
        return str.TakeStr();
```

toolchain/base/kind_switch_test.cpp

@@ -16,36 +16,33 @@ namespace {
 
 TEST(KindSwitch, Variant) {
   auto f = [](std::variant<int, float, char> v) -> std::string {
-    RawStringOstream str;
     CARBON_KIND_SWITCH(v) {
-      case CARBON_KIND(int n):
-        str << "int = " << n;
-        return str.TakeStr();
-      case CARBON_KIND(float f):
-        str << "float = " << f;
-        return str.TakeStr();
-      case CARBON_KIND(char c):
-        str << "char = " << c;
-        return str.TakeStr();
+      case CARBON_KIND(int n): {
+        return llvm::formatv("int = {0}", n);
+      }
+      case CARBON_KIND(float f): {
+        return llvm::formatv("float = {0}", f);
+      }
+      case CARBON_KIND(char c): {
+        return llvm::formatv("char = {0}", c);
+      }
     }
   };
 
   EXPECT_EQ(f(int{1}), "int = 1");
-  EXPECT_EQ(f(float{2}), "float = 2.000000e+00");
+  EXPECT_EQ(f(float{2}), "float = 2.00");
   EXPECT_EQ(f(char{'h'}), "char = h");
 }
 
 TEST(KindSwitch, VariantUnusedValue) {
   auto f = [](std::variant<int, float> v) -> std::string {
-    RawStringOstream str;
     CARBON_KIND_SWITCH(v) {
-      case CARBON_KIND(int n):
-        str << "int = " << n;
-        return str.TakeStr();
+      case CARBON_KIND(int n): {
+        return llvm::formatv("int = {0}", n);
+      }
       case CARBON_KIND(float _):
         // The float value is not used, we see that using `_` works.
-        str << "float";
-        return str.TakeStr();
+        return "float";
     }
   };