Add a small lowering test with an imported argument. (#4529)

We don't have much import coverage at present, but this seems worth a
tiny bit of validation. I believe the LLVM IR is currently correct.

Author: jonmeow

toolchain/lower/testdata/packages/cross_package_call.carbon

@@ -12,13 +12,18 @@
 
 package A;
 namespace NS;
+
 fn NS.F() {}
 
+fn NS.G(x: i32) -> i32 { return x; }
+
 // --- b.carbon
 
 import A;
 
-fn G() { A.NS.F(); }
+fn F() { A.NS.F(); }
+
+fn G() -> i32 { return A.NS.G(1); }
 
 // CHECK:STDOUT: ; ModuleID = 'a.carbon'
 // CHECK:STDOUT: source_filename = "a.carbon"
@@ -28,37 +33,55 @@ fn G() { A.NS.F(); }
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: define i32 @_CG.NS.A(i32 %x) !dbg !8 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   ret i32 %x, !dbg !9
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
 // CHECK:STDOUT: !llvm.dbg.cu = !{!2}
 // CHECK:STDOUT:
 // CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "a.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.NS.A", scope: null, file: !3, line: 4, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.NS.A", scope: null, file: !3, line: 5, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
-// CHECK:STDOUT: !7 = !DILocation(line: 4, column: 1, scope: !4)
+// CHECK:STDOUT: !7 = !DILocation(line: 5, column: 1, scope: !4)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.NS.A", scope: null, file: !3, line: 7, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = !DILocation(line: 7, column: 26, scope: !8)
 // CHECK:STDOUT: ; ModuleID = 'b.carbon'
 // CHECK:STDOUT: source_filename = "b.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @_CG.Main() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   call void @_CF.NS.A(), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: declare void @_CF.NS.A()
 // CHECK:STDOUT:
+// CHECK:STDOUT: define i32 @_CG.Main() !dbg !9 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %G.call = call i32 @_CG.NS.A(i32 1), !dbg !10
+// CHECK:STDOUT:   ret i32 %G.call, !dbg !11
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare i32 @_CG.NS.A(i32)
+// CHECK:STDOUT:
 // CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
 // CHECK:STDOUT: !llvm.dbg.cu = !{!2}
 // CHECK:STDOUT:
 // CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "b.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 4, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 4, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 4, column: 10, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 4, column: 1, scope: !4)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 6, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = !DILocation(line: 6, column: 24, scope: !9)
+// CHECK:STDOUT: !11 = !DILocation(line: 6, column: 17, scope: !9)