test/codegen: test inter-crate linkage with static relocation

Add a codegen-test that verifies inter-crate linkage with the static
relocation model. We expect all symbols that are part of a rust
compilation to end up in the same DSO, thus we expect `dso_local`
annotations.

src/test/codegen/auxiliary/extern_decl.rs

@@ -0,0 +1,11 @@
+// Auxiliary crate that exports a function and static. Both always
+// evaluate to `71`. We force mutability on the static to prevent
+// it from being inlined as constant.
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn extern_fn() -> u8 { unsafe { extern_static } }
+
+#[no_mangle]
+pub static mut extern_static: u8 = 71;

src/test/codegen/static-relocation-model-msvc.rs

@@ -0,0 +1,26 @@
+// Verify linkage of external symbols in the static relocation model on MSVC.
+//
+// compile-flags: -O -C relocation-model=static
+// aux-build: extern_decl.rs
+// only-x86_64-pc-windows-msvc
+
+#![crate_type = "rlib"]
+
+extern crate extern_decl;
+
+// The `extern_decl` definitions are imported from a statically linked rust
+// crate, thus they are expected to be marked `dso_local` without `dllimport`.
+//
+// The `access_extern()` symbol is from this compilation unit, thus we expect
+// it to be marked `dso_local` as well, given the static relocation model.
+//
+// CHECK: @extern_static = external dso_local local_unnamed_addr global i8
+// CHECK: define dso_local i8 @access_extern() {{.*}}
+// CHECK: declare dso_local i8 @extern_fn() {{.*}}
+
+#[no_mangle]
+pub fn access_extern() -> u8 {
+    unsafe {
+        extern_decl::extern_fn() + extern_decl::extern_static
+    }
+}