Swift: Use absl::bit_width to calculate TRAP label size.

It's not much cleaner due to arithmetic to convert truncating division to a ceiling, but has two advantages:
 1. It doesn't suffer from rounding issues with large TRAP labels. This is largely theoretical, but does let us handle `undefined` uniformly.
 2. It should be much faster (using LZCNT/BSR instead of floating point arithmetic). This is probably not a performance bottleneck, so *shrug*.

swift/extractor/trap/BUILD.bazel

@@ -50,5 +50,6 @@ swift_cc_library(
     deps = [
         "//swift/extractor/infra/file",
         "//swift/extractor/infra/log",
+        "@absl//absl/numeric:bits",
     ],
 )

swift/extractor/trap/TrapLabel.h

@@ -5,6 +5,7 @@
 #include <iostream>
 #include <string>
 #include <vector>
+#include "absl/numeric/bits.h"
 #include <binlog/binlog.hpp>
 #include <cmath>
 #include <charconv>
@@ -53,10 +54,9 @@ class UntypedTrapLabel {
 
  private:
   size_t strSize() const {
-    if (id_ == undefined) return 17;  // #ffffffffffffffff
-    if (id_ == 0) return 2;           // #0
-    // TODO: use absl::bit_width or C+20 std::bit_width instead of this ugly formula
-    return /* # */ 1 + /* hex digits */ static_cast<size_t>(ceil(log2(id_ + 1) / 4));
+    if (id_ == 0) return 2;  // #0
+    // Number of hex digits is ceil(bit_width(id) / 4), but C++ integer division can only do floor.
+    return /* # */ 1 + /* hex digits */ 1 + (absl::bit_width(id_) - 1) / 4;
   }
 };