[Sema] Reword unused lambda capture warning

[malcolm.parsons]

Summary

The warning doesn't know why the variable was looked up but not
odr-used, so reword it to not claim that it was used in an unevaluated
context.

[aaron.ballman]

LGTM!

include/clang/Basic/DiagnosticSemaKinds.td

def warn_unneeded_static_internal_decl : Warning<
   "should be declared 'static inline'">,
   InGroup<UnneededInternalDecl>, DefaultIgnore;
 def warn_unneeded_member_function : Warning<
   "member function %0 is not needed and will not be emitted">,
   InGroup<UnneededMemberFunction>, DefaultIgnore;
 def warn_unused_private_field: Warning<"private field %0 is not used">,
   InGroup<UnusedPrivateField>, DefaultIgnore;
 def warn_unused_lambda_capture: Warning<"lambda capture %0 is not "
-  "%select{used|required to be captured for use in an unevaluated context}1">,
+  "%select{used|required to be captured for this use}1">,
   InGroup<UnusedLambdaCapture>, DefaultIgnore;
 
 def warn_parameter_size: Warning<
   "%0 is a large (%1 bytes) pass-by-value argument; "
   "pass it by reference instead ?">, InGroup<LargeByValueCopy>;
 def warn_return_value_size: Warning<
   "return value of %0 is a large (%1 bytes) pass-by-value object; "
   "pass it by reference instead ?">, InGroup<LargeByValueCopy>;

test/SemaCXX/warn-unused-lambda-capture.cpp

 };
 
 int side_effect() {
   return 42;
 }
 
 void test() {
   int i = 0;
+  const int k = 0;
 
   auto captures_nothing = [] {};
 
   auto captures_nothing_by_value = [=] {};
   auto captures_nothing_by_reference = [&] {};
 
   auto implicit_by_value = [=]() mutable { i++; };
   auto implicit_by_reference = [&] { i++; };
 
   auto explicit_by_value_used = [i] { return i + 1; };
   auto explicit_by_value_used_void = [i] { (void)i; };
   auto explicit_by_value_unused = [i] {}; // expected-warning{{lambda capture 'i' is not used}}
-  auto explicit_by_value_unused_sizeof = [i] { return sizeof(i); }; // expected-warning{{lambda capture 'i' is not required to be captured for use in an unevaluated context}}
-  auto explicit_by_value_unused_decltype = [i] { decltype(i) j = 0; }; // expected-warning{{lambda capture 'i' is not required to be captured for use in an unevaluated context}}
+  auto explicit_by_value_unused_sizeof = [i] { return sizeof(i); }; // expected-warning{{lambda capture 'i' is not required to be captured for this use}}
+  auto explicit_by_value_unused_decltype = [i] { decltype(i) j = 0; }; // expected-warning{{lambda capture 'i' is not required to be captured for this use}}
+  auto explicit_by_value_unused_const = [k] { return k + 1; };         // expected-warning{{lambda capture 'k' is not required to be captured for this use}}
 
   auto explicit_by_reference_used = [&i] { i++; };
   auto explicit_by_reference_unused = [&i] {}; // expected-warning{{lambda capture 'i' is not used}}
 
   auto explicit_initialized_reference_used = [&j = i] { return j + 1; };
   auto explicit_initialized_reference_unused = [&j = i]{}; // expected-warning{{lambda capture 'j' is not used}}
 
   auto explicit_initialized_value_used = [j = 1] { return j + 1; };
void test() {
     auto explicit_this_unused = [this] {}; // expected-warning{{lambda capture 'this' is not used}}
   }
   int i;
 };
 
 template <typename T>
 void test_templated() {
   int i = 0;
+  const int k = 0;
 
   auto captures_nothing = [] {};
 
   auto captures_nothing_by_value = [=] {};
   auto captures_nothing_by_reference = [&] {};
 
   auto implicit_by_value = [=]() mutable { i++; };
   auto implicit_by_reference = [&] { i++; };
 
   auto explicit_by_value_used = [i] { return i + 1; };
   auto explicit_by_value_used_void = [i] { (void)i; };
   auto explicit_by_value_unused = [i] {}; // expected-warning{{lambda capture 'i' is not used}}
-  auto explicit_by_value_unused_sizeof = [i] { return sizeof(i); }; // expected-warning{{lambda capture 'i' is not required to be captured for use in an unevaluated context}}
+  auto explicit_by_value_unused_sizeof = [i] { return sizeof(i); }; // expected-warning{{lambda capture 'i' is not required to be captured for this use}}
   auto explicit_by_value_unused_decltype = [i] { decltype(i) j = 0; }; // expected-warning{{lambda capture 'i' is not used}}
+  auto explicit_by_value_unused_const = [k] { return k + 1; };         // expected-warning{{lambda capture 'k' is not required to be captured for this use}}
 
   auto explicit_by_reference_used = [&i] { i++; };
   auto explicit_by_reference_unused = [&i] {}; // expected-warning{{lambda capture 'i' is not used}}
 
   auto explicit_initialized_reference_used = [&j = i] { return j + 1; };
   auto explicit_initialized_reference_unused = [&j = i]{}; // expected-warning{{lambda capture 'j' is not used}}
 
   auto explicit_initialized_value_used = [j = 1] { return j + 1; };