[clang] Add tests for (const) weak variables

[wanders]

Summary

This adds tests checking the behavior of const variables declared with
weak attribute.

Both checking that they can not be used in places where a constant
expression is required and that a dynamic initializer is emitted when
used as an initializer expression.

[aaron.ballman]

Thanks for looking to improve our test coverage here! However, precommit CI looks to be failing:

********************
Failed Tests (1):
  Clang :: CodeGenCXX/weak-init.cpp

[wanders]

Added explicit triple to hopefully make it pass on win.
Made the new files clang-format clean.

[wanders]

In D126578#3543994, @aaron.ballman wrote:

Thanks for looking to improve our test coverage here! However, precommit CI looks to be failing:

Thanks for notifying me. It should be green now.

[aaron.ballman]

LGTM!

clang/test/CodeGenCXX/weak-init.cpp

+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -emit-llvm %s -o - | FileCheck %s
+
+extern const int W __attribute__((weak)) = 99;
+const int S = 77;
+
+// CHECK: @C1 = {{.*}} 77
+extern const int C1 = S;
+
+// CHECK: %0 = load {{.*}} @W
+// CHECK-NEXT: store {{.*}} %0, {{.*}} @C2
+extern const int C2 = W;

clang/test/SemaCXX/weak-init.cpp

+// RUN: %clang_cc1 %s -verify -fsyntax-only
+
+extern const int W1 __attribute__((weak)) = 10; // expected-note {{declared here}}
+
+static_assert(W1 == 10, ""); // expected-error   {{static_assert expression is not an integral constant expression}}
+                             // expected-note@-1 {{initializer of weak variable 'W1' is not considered constant because it may be different at runtime}}
+
+extern const int W2 __attribute__((weak)) = 20;
+
+int S2[W2]; // expected-error {{variable length array declaration not allowed at file scope}}
+
+extern const int W3 __attribute__((weak)) = 30; // expected-note {{declared here}}
+
+constexpr int S3 = W3; // expected-error   {{constexpr variable 'S3' must be initialized by a constant expression}}
+                       // expected-note@-1 {{initializer of weak variable 'W3' is not considered constant because it may be different at runtime}}