[libcxx] [test] Avoid MSVC truncation warnings.

MSVC emits "warning C4244: 'initializing': conversion from 'int'
to 'short', possible loss of data" when it sees pair<Whatever, short>
constructed from (whatever, 4), because int is being truncated to
short within pair's constructor. (The compiler doesn't take into
account the fact that 4 is a literal at the callsite; it generates
this warning when the constructor is instantiated, because it might
be called with a runtime-valued int that would actually truncate.)

Instead of static_cast<short>, we can simply change short to int
in these tests, without affecting the pair operations that they're
trying to test: move assignment, convert copy construction, and
convert move construction.

Fixes D45016.

llvm-svn: 329973