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/array/array_in_place.carbon

@@ -16,6 +16,7 @@ fn G() {
   var _: array((i32, i32, i32), 2) = (F(), F());
 }
 
+// CHECK:STDOUT: ; ---
 // CHECK:STDOUT: ; ModuleID = 'array_in_place.carbon'
 // CHECK:STDOUT: source_filename = "array_in_place.carbon"
 // CHECK:STDOUT:
@@ -91,3 +92,4 @@ fn G() {
 // CHECK:STDOUT: !27 = !{!28}
 // CHECK:STDOUT: !28 = !DILocalVariable(arg: 1, scope: !26, type: !22)
 // CHECK:STDOUT: !29 = !DILocation(line: 16, column: 3, scope: !26)
+// CHECK:STDOUT: