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/debug/nodebug.carbon

@@ -14,6 +14,7 @@
 fn F() {
 }
 
+// CHECK:STDOUT: ; ---
 // CHECK:STDOUT: ; ModuleID = 'nodebug.carbon'
 // CHECK:STDOUT: source_filename = "nodebug.carbon"
 // CHECK:STDOUT:
@@ -24,3 +25,4 @@ fn F() {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: attributes #0 = { nounwind }
+// CHECK:STDOUT: