Add code to honor the match_not_bol and match_not_eol regex flats. Fixes PR#22651. Thanks to Jim Porter for the report and suggested fix.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@232733 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/regex b/include/regex
index fb0f7d8..bd48fb8 100644
--- a/include/regex
+++ b/include/regex
@@ -1947,7 +1947,8 @@
 void
 __l_anchor<_CharT>::__exec(__state& __s) const
 {
-    if (__s.__at_first_ && __s.__current_ == __s.__first_)
+    if (__s.__at_first_ && __s.__current_ == __s.__first_ &&
+        !(__s.__flags_ & regex_constants::match_not_bol))
     {
         __s.__do_ = __state::__accept_but_not_consume;
         __s.__node_ = this->first();
@@ -1981,7 +1982,8 @@
 void
 __r_anchor<_CharT>::__exec(__state& __s) const
 {
-    if (__s.__current_ == __s.__last_)
+    if (__s.__current_ == __s.__last_ &&
+        !(__s.__flags_ & regex_constants::match_not_eol))
     {
         __s.__do_ = __state::__accept_but_not_consume;
         __s.__node_ = this->first();