[Sema] Improve test coverage of narrowing conversion diagnostics

[miyuki]

Summary

This patch adds tests of narrowing conversion diagnostics for the
'unscoped enum -> integer' case.

[rogfer01]

Looks good to me. Thanks!

test/CXX/dcl.decl/dcl.init/dcl.init.list/p7-0x.cpp

 // * from an integer type or unscoped enumeration type to an integer type that
 //   cannot represent all the values of the original type, except where the
 //   source is a constant expression and the actual value after conversion will
 //   fit into the target type and will produce the original value when converted
 //   back to the original type.
 void shrink_int() {
   // Not a constant expression.
   short s = 1;
+  UnscopedEnum e = EnumVal;
   unsigned short us = 1;
   Agg<char> c1 = {s};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
+  Agg<char> c2 = {e};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
   Agg<unsigned short> s1 = {s};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
   Agg<short> s2 = {us};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
 
   // "that cannot represent all the values of the original type" means that the
   // validity of the program depends on the relative sizes of integral types.
   // This test compiles with -m64, so sizeof(int)<sizeof(long)==sizeof(long
   // long).
   long l1 = 1;
   Agg<int> i1 = {l1};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
+  Agg<int> i2 = {e};  // OK
   long long ll = 1;
   Agg<long> l2 = {ll};  // OK
 
   // Constants.
-  Agg<char> c2 = {127};  // OK
-  Agg<char> c3 = {300};  // expected-error {{ cannot be narrowed }} expected-note {{silence}} expected-warning {{changes value}}
-
-  Agg<int> i2 = {0x7FFFFFFFU};  // OK
-  Agg<int> i3 = {0x80000000U};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
-  Agg<unsigned int> i4 = {-0x80000000L};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
+  Agg<char> c3 = {127};  // OK
+  Agg<char> c4 = {300};  // expected-error {{ cannot be narrowed }} expected-note {{silence}} expected-warning {{changes value}}
+  Agg<char> c5 = {EnumVal};  // expected-error {{ cannot be narrowed }} expected-note {{silence}} expected-warning {{changes value}}
+
+  Agg<int> i3 = {0x7FFFFFFFU};  // OK
+  Agg<int> i4 = {EnumVal};  // OK
+  Agg<int> i5 = {0x80000000U};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
+  Agg<unsigned int> i6 = {-0x80000000L};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
 
   // Bool is also an integer type, but conversions to it are a different AST
   // node.
   Agg<bool> b1 = {0};  // OK
   Agg<bool> b2 = {1};  // OK
   Agg<bool> b3 = {-1};  // expected-error {{ cannot be narrowed }} expected-note {{silence}}
 
   // Conversions from pointers to booleans aren't narrowing conversions.