Update files and clang-tidy config to pass with clang-tidy-20 (#4691)

Disables three new warnings because they lean more towards style
conflicts than fixes. I've brought these up on #style.

Other than that, mostly fixing basic issues, and things that
clang-tidy-20 seems to fire where clang-tiday-16 didn't. One particular
curious case is `llvm::StringLiteral::data()` uses, which are flagged as
not strictly null-terminated; I'm switching to `const char*` in those
spots which matches `llvm::formatv`'s format argument, but feels worse.

I'm removing `run_clang_tidy.py` here because I'm observing it give
fewer warnings than `bazel build --config=clang-tidy -k
//toolchain/...`. The latter matches how we enforce in GitHub actions
(and also caches results, and suppresses output for files that have no
issues), so I'm dropping the bespoke script.

toolchain/lex/tokenized_buffer_benchmark.cpp

@@ -610,6 +610,7 @@ template <const DispatchTableT& Table>
 auto BasicDispatch(ssize_t& index, const char* text, char* buffer) -> void {
   *buffer = text[index];
   ++index;
+  // NOLINTNEXTLINE(readability-avoid-return-with-void-value): For musttail.
   [[clang::musttail]] return Table[static_cast<unsigned char>(text[index])](
       index, text, buffer);
 }
@@ -620,6 +621,7 @@ auto SpecializedDispatch(ssize_t& index, const char* text, char* buffer)
   CARBON_CHECK(C == text[index]);
   *buffer = C;
   ++index;
+  // NOLINTNEXTLINE(readability-avoid-return-with-void-value): For musttail.
   [[clang::musttail]] return Table[static_cast<unsigned char>(text[index])](
       index, text, buffer);
 }