[libc++][test] Miscellaneous MSVC cleanups

* Silence unused-local-typedef warnings: `map.cons/assign_initializer_list.pass.cpp` (and the `set.cons` variant) uses a local typedef only within `LIBCPP_ASSERT`s, so clang diagnoses it as unused when testing non-libc++.
* Add missing include: `c.math/abs.pass.cpp` uses `std::numeric_limits` but failed to `#include <limits>`.
* Don't test non-type: A "recent" change to `meta.trans.other/underlying_type.pass.cpp` unconditionally tests the type `F` which is conditionally defined.
* Use `hash<long long>` instead of `hash<short>` with `int` in `unordered_meow` deduction guide tests to avoid truncation warnings.
* Convert `3.14` explicitly in `midpoint.float.pass` since MSVC incorrectly diagnoses `float meow = 3.14;` as truncating.

Differential Revision: https://reviews.llvm.org/D68681

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374248 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/numerics/c.math/abs.pass.cpp

@@ -9,6 +9,7 @@
 #include <assert.h>
 #include <cmath>
 #include <cstdint>
+#include <limits>
 #include <type_traits>
 
 #include "test_macros.h"
@@ -75,4 +76,3 @@ int main(int, char**)
 
     return 0;
 }
-