[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss...

[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss of data", part 1/7.

Given `std::basic_streambuf<CharT>::int_type __c`, `std::basic_string<CharT> str_`,
and having checked `__c != std::basic_streambuf<CharT>::traits_type::eof()` (substituting typedefs
for clarity), the line `str_.push_back(__c);` is safe according to humans, but truncates according
to compilers. `str_.push_back(static_cast<CharT>(__c));` avoids that problem.

Fixes D27538.

llvm-svn: 289105