[libcxx] [test] Fix MSVC warnings and errors.

test/std/numerics/numeric.ops/exclusive.scan/exclusive_scan.pass.cpp
test/std/numerics/numeric.ops/exclusive.scan/exclusive_scan_init_op.pass.cpp
test/std/numerics/numeric.ops/inclusive.scan/inclusive_scan.pass.cpp
test/std/numerics/numeric.ops/inclusive.scan/inclusive_scan_op.pass.cpp
test/std/numerics/numeric.ops/inclusive.scan/inclusive_scan_op_init.pass.cpp
test/std/numerics/numeric.ops/transform.exclusive.scan/transform_exclusive_scan_init_bop_uop.pass.cpp
test/std/numerics/numeric.ops/transform.inclusive.scan/transform_inclusive_scan_bop_uop.pass.cpp
test/std/numerics/numeric.ops/transform.inclusive.scan/transform_inclusive_scan_bop_uop_init.pass.cpp
Fix MSVC x64 truncation warnings.
warning C4267: conversion from 'size_t' to 'int', possible loss of data

test/std/strings/basic.string/string.modifiers/string_append/push_back.pass.cpp
Fix MSVC uninitialized memory warning.
warning C6001: Using uninitialized memory 'vl'.

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp
Include <cassert> for the assert() macro.

Fixes D43273.

llvm-svn: 326120