[dataflow][nfc] Fix u8 string usage with c++20 (#84291)

Clang returns an error when compiling this file with c++20
```
error: ISO C++20 does not permit initialization of char array with UTF-8 string literal
```
It seems like c++20 treats u8strings differently than strings (probably
needs char8_t).
Make this a string to fix the error.

clang/lib/Analysis/FlowSensitive/HTMLLogger.cpp

@@ -500,7 +500,7 @@ class HTMLLogger : public Logger {
     for (unsigned I = 0; I < CFG.getNumBlockIDs(); ++I) {
       std::string Name = blockID(I);
       // Rightwards arrow, vertical line
-      char ConvergenceMarker[] = u8"\\n\u2192\u007c";
+      char ConvergenceMarker[] = "\\n\u2192\u007c";
       if (BlockConverged[I])
         Name += ConvergenceMarker;
       GraphS << "  " << blockID(I) << " [id=" << blockID(I) << " label=\""