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/struct/nested_struct.carbon

@@ -15,6 +15,7 @@ fn Run() -> i32 {
   return 0;
 }
 
+// CHECK:STDOUT: ; ---
 // CHECK:STDOUT: ; ModuleID = 'nested_struct.carbon'
 // CHECK:STDOUT: source_filename = "nested_struct.carbon"
 // CHECK:STDOUT:
@@ -38,3 +39,4 @@ fn Run() -> i32 {
 // CHECK:STDOUT: !6 = !{!7}
 // CHECK:STDOUT: !7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
 // CHECK:STDOUT: !8 = !DILocation(line: 15, column: 3, scope: !4)
+// CHECK:STDOUT: