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/function_param.carbon

@@ -18,6 +18,7 @@ fn G() -> i32 {
   return F((1, 2, 3), 1);
 }
 
+// CHECK:STDOUT: ; ---
 // CHECK:STDOUT: ; ModuleID = 'function_param.carbon'
 // CHECK:STDOUT: source_filename = "function_param.carbon"
 // CHECK:STDOUT:
@@ -98,3 +99,4 @@ fn G() -> i32 {
 // CHECK:STDOUT: !29 = !{!30}
 // CHECK:STDOUT: !30 = !DILocalVariable(arg: 1, scope: !26, type: !8)
 // CHECK:STDOUT: !31 = !DILocation(line: 18, column: 12, scope: !26)
+// CHECK:STDOUT: