Fix LWG#2244: basic_istream::seekg

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@251246 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/istream b/include/istream
index 168a4d0..0bcc7ee 100644
--- a/include/istream
+++ b/include/istream
@@ -1407,6 +1407,7 @@
     try
     {
 #endif  // _LIBCPP_NO_EXCEPTIONS
+        this->clear(this->rdstate() & ~ios_base::eofbit);
         sentry __sen(*this, true);
         if (__sen)
         {