Add separators between files in LLVM IR dumps (#7463)

Each file dump now starts with a `; ---` comment and ends with a blank
line. This makes it easier to visually scan the dump for a file of
interest. The comment format is somewhat arbitrary; I chose `---` to
align with the `--- filename.carbon` separator in SemIR dumps, but
without the filename, because that appears on each of the next two lines
already.

toolchain/lower/testdata/interop/cpp/array.carbon

@@ -28,6 +28,7 @@ int H() {
 }
 ''';
 
+// CHECK:STDOUT: ; ---
 // CHECK:STDOUT: ; ModuleID = 'array_value_param.carbon'
 // CHECK:STDOUT: source_filename = "array_value_param.carbon"
 // CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
@@ -136,3 +137,4 @@ int H() {
 // CHECK:STDOUT: !31 = !DILocalVariable(arg: 1, scope: !27, type: !22)
 // CHECK:STDOUT: !32 = !DILocalVariable(arg: 2, scope: !27, type: !21)
 // CHECK:STDOUT: !33 = !DILocation(line: 5, column: 1, scope: !27)
+// CHECK:STDOUT: