[libcxx] [test] D27267: Fix MSVC x64 warning C4267 "conversion from 'size_t' to ...

[libcxx] [test] D27267: Fix MSVC x64 warning C4267 "conversion from 'size_t' to 'int' [or 'unsigned int'], possible loss of data", part 1/4.

Replace "int n = str_.size();" with "int n = static_cast<int>(str_.size());".

int is the correct type to use, because we're eventually calling
"base::pbump(n+1);" where base is std::basic_streambuf.
N4606 27.6.3.3.3 [streambuf.put.area]/4 declares: "void pbump(int n);"

llvm-svn: 288751