[clang-format] [NFC] Restructure getLineCommentIndentPrefix

When sorting the known prefixes after length the if in the loop will hit
at most once, so we can return from there.

Also replace the inner loop with an algorithm, that makes it more
readable.

Differential Revision: https://reviews.llvm.org/D95081

Author: HazardyKnusperkeks

clang/lib/Format/BreakableToken.cpp

@@ -42,24 +42,26 @@ static bool IsBlank(char C) {
 static StringRef getLineCommentIndentPrefix(StringRef Comment,
                                             const FormatStyle &Style) {
   static constexpr StringRef KnownCStylePrefixes[] = {"///<", "//!<", "///",
-                                                      "//",   "//!",  "//:"};
-  static constexpr StringRef KnownTextProtoPrefixes[] = {"//", "#", "##", "###",
-                                                         "####"};
+                                                      "//!",  "//:",  "//"};
+  static constexpr StringRef KnownTextProtoPrefixes[] = {"####", "###", "##",
+                                                         "//", "#"};
   ArrayRef<StringRef> KnownPrefixes(KnownCStylePrefixes);
   if (Style.Language == FormatStyle::LK_TextProto)
     KnownPrefixes = KnownTextProtoPrefixes;
 
-  StringRef LongestPrefix;
+  assert(std::is_sorted(KnownPrefixes.begin(), KnownPrefixes.end(),
+                        [](StringRef Lhs, StringRef Rhs) noexcept {
+                          return Lhs.size() > Rhs.size();
+                        }));
+
   for (StringRef KnownPrefix : KnownPrefixes) {
     if (Comment.startswith(KnownPrefix)) {
-      size_t PrefixLength = KnownPrefix.size();
-      while (PrefixLength < Comment.size() && Comment[PrefixLength] == ' ')
-        ++PrefixLength;
-      if (PrefixLength > LongestPrefix.size())
-        LongestPrefix = Comment.substr(0, PrefixLength);
+      const auto PrefixLength =
+          Comment.find_first_not_of(' ', KnownPrefix.size());
+      return Comment.substr(0, PrefixLength);
     }
   }
-  return LongestPrefix;
+  return {};
 }
 
 static BreakableToken::Split