[libcxx] [test] Fix MSVC x64 truncation warning.

warning C4267: 'argument': conversion from 'size_t' to 'int', possible loss of data

Requesting post-commit review.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@331575 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/input.output/iostream.format/input.streams/istream.formatted/istream_extractors/streambuf.pass.cpp

@@ -50,7 +50,7 @@ protected:
                 str_.resize(str_.capacity());
                 base::setp(const_cast<CharT*>(str_.data()),
                            const_cast<CharT*>(str_.data() + str_.size()));
-                base::pbump(n+1);
+                base::pbump(static_cast<int>(n+1));
             }
             return ch;
         }