[libcxx] [test] Fix string_view tests.

test/std/strings/string.view/string.view.ops/compare.pointer_size.pass.cpp
Passing -1 to size_t triggers signed/unsigned mismatch warnings because it's
a value-modifying conversion. Add static_cast<size_t> to soothe the compiler.
(This file refers to size_t unqualified.)

test/std/strings/string.view/string.view.ops/substr.pass.cpp
Add <algorithm> for std::min() and <stdexcept> for std::out_of_range.

N4618 21.4.2.4 [string.view.access]/1: "Requires: pos < size()."
/4: "[ Note: Unlike basic_string::operator[], basic_string_view::operator[](size())
has undefined behavior instead of returning charT(). -end note ]"

Fixes D27633.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@289283 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/string.view/string.view.ops/substr.pass.cpp

@@ -16,6 +16,8 @@
 // Returns: basic_string_view(data()+pos, rlen).
 
 #include <string_view>
+#include <algorithm>
+#include <stdexcept>
 #include <cassert>
 
 #include "test_macros.h"
@@ -39,7 +41,7 @@ void test1(std::basic_string_view<CharT> sv, size_t n, size_t pos) {
 #endif
     const size_t rlen = std::min(n, sv.size() - pos);
     assert (sv1.size() == rlen);
-    for (size_t i = 0; i <= rlen; ++i)
+    for (size_t i = 0; i < rlen; ++i)
         assert(sv[pos+i] == sv1[i]);
 }