Add bit packing to NodeImpl (#4651)

Just a small packing optimization. We currently have 222 `NodeKinds`, so
this reduces us to just 30ish more we can add without needing to pack
more. However, if we did, there would be a couple options for bringing
the count down by reusing `NodeKinds` and disambiguating based on the
token kind (the 29 infix operators as an example). Or we could just undo
this.

I'm expecting this to yield a small improvement. I'll see if I can get
better numbers since my machine's not really reliable, but here are some
basic values.

Also suggesting to draw the use of `::RawEnumType` for `TokenKind`,
since bit packing appears to work without it. Hoping the `static_assert`
is easier for people to understand the size of the field.

With the change:

```
----------------------------------------------------------------------------------------------------------------------------
Benchmark                                                 Time             CPU   Iterations      Bytes      Lines     Tokens
----------------------------------------------------------------------------------------------------------------------------
BM_CompileAPIFileDenseDecls<Phase::Parse>/256         50399 ns        50359 ns        14336 104.588M/s 3.87217M/s 21.8629M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/1024       237823 ns       237629 ns         3072 136.721M/s 4.11986M/s 24.2058M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/4096       997645 ns       996771 ns          768 142.343M/s 4.04105M/s 23.9363M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/16384     4020308 ns      4018319 ns          192 152.041M/s 4.05966M/s 24.0874M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/65536    16691390 ns     16683058 ns           48 151.317M/s 3.92374M/s 23.2936M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/262144   75265735 ns     75233476 ns            8 135.842M/s 3.48421M/s 20.6862M/s
```

Without the change:
```
----------------------------------------------------------------------------------------------------------------------------
Benchmark                                                 Time             CPU   Iterations      Bytes      Lines     Tokens
----------------------------------------------------------------------------------------------------------------------------
BM_CompileAPIFileDenseDecls<Phase::Parse>/256         51515 ns        51480 ns        13312 102.312M/s 3.78789M/s  21.387M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/1024       241040 ns       240900 ns         3072 134.865M/s 4.06392M/s 23.8771M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/4096       985593 ns       984657 ns          768 144.094M/s 4.09077M/s 24.2308M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/16384     4109327 ns      4105496 ns          192 148.813M/s 3.97345M/s  23.576M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/65536    17459655 ns     17446006 ns           48   144.7M/s 3.75215M/s  22.275M/s
BM_CompileAPIFileDenseDecls<Phase::Parse>/262144   80802815 ns     80737489 ns            8 126.581M/s 3.24668M/s  19.276M/s
```

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/lex/tokenized_buffer_test.cpp

@@ -1160,9 +1160,9 @@ TEST_F(LexerTest, DiagnosticFileTooLarge) {
     input += "{}\n";
   }
   EXPECT_CALL(consumer,
-              HandleDiagnostic(IsSingleDiagnostic(
-                  DiagnosticKind::TooManyTokens, DiagnosticLevel::Error,
-                  TokenizedBuffer::MaxTokens / 2, 1, _)));
+              HandleDiagnostic(IsSingleDiagnostic(DiagnosticKind::TooManyTokens,
+                                                  DiagnosticLevel::Error,
+                                                  TokenIndex::Max / 2, 1, _)));
   compile_helper_.GetTokenizedBuffer(input, &consumer);
 }