[clang-format] [NFC] Use some constexpr StringRef instead of const char*.

[curdeius]

LGTM.

clang/lib/Format/BreakableToken.cpp

 #include "llvm/Support/Debug.h"
 #include <algorithm>
 
 #define DEBUG_TYPE "format-token-breaker"
 
 namespace clang {
 namespace format {
 
-static const char *const Blanks = " \t\v\f\r";
+static constexpr StringRef Blanks = " \t\v\f\r";
 static bool IsBlank(char C) {
   switch (C) {
   case ' ':
   case '\t':
   case '\v':
   case '\f':
   case '\r':
     return true;
   default:
     return false;
   }
 }
 
 static StringRef getLineCommentIndentPrefix(StringRef Comment,
                                             const FormatStyle &Style) {
-  static const char *const KnownCStylePrefixes[] = {"///<", "//!<", "///",
+  static constexpr StringRef KnownCStylePrefixes[] = {"///<", "//!<", "///",
                                                       "//",   "//!",  "//:"};
-  static const char *const KnownTextProtoPrefixes[] = {"//", "#", "##", "###",
+  static constexpr StringRef KnownTextProtoPrefixes[] = {"//", "#", "##", "###",
                                                          "####"};
-  ArrayRef<const char *> KnownPrefixes(KnownCStylePrefixes);
+  ArrayRef<StringRef> KnownPrefixes(KnownCStylePrefixes);
   if (Style.Language == FormatStyle::LK_TextProto)
     KnownPrefixes = KnownTextProtoPrefixes;
 
   StringRef LongestPrefix;
   for (StringRef KnownPrefix : KnownPrefixes) {
     if (Comment.startswith(KnownPrefix)) {
       size_t PrefixLength = KnownPrefix.size();
       while (PrefixLength < Comment.size() && Comment[PrefixLength] == ' ')