[clang][doc] Don't escape _ in .rst files. (#65277)

The current generated ClangCommandLineReference.rst unconditionally
escapes underscores. This leads odd output on the website
https://clang.llvm.org/docs/ClangCommandLineReference.html

For example

   -fchar8\_t, -fno-char8\_t

Whether an underscore should be escaped depends on the state. Currently
the escape routine doesn't keep track of the state and currently
underscores are not used in places where they need to be escaped.
Therefore remove the underscore from the list of escaped characters.

Author: mordante

clang/utils/TableGen/ClangOptionDocEmitter.cpp

@@ -194,7 +194,7 @@ unsigned getNumArgsForKind(Record *OptionKind, const Record *Option) {
 std::string escapeRST(StringRef Str) {
   std::string Out;
   for (auto K : Str) {
-    if (StringRef("`*|_[]\\").count(K))
+    if (StringRef("`*|[]\\").count(K))
       Out.push_back('\\');
     Out.push_back(K);
   }