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/pointer/basic.carbon

@@ -19,6 +19,7 @@ fn F() -> i32 {
   return G(&n);
 }
 
+// CHECK:STDOUT: ; ---
 // CHECK:STDOUT: ; ModuleID = 'basic.carbon'
 // CHECK:STDOUT: source_filename = "basic.carbon"
 // CHECK:STDOUT:
@@ -80,3 +81,4 @@ fn F() -> i32 {
 // CHECK:STDOUT: !22 = !{!23}
 // CHECK:STDOUT: !23 = !DILocalVariable(arg: 1, scope: !19, type: !7)
 // CHECK:STDOUT: !24 = !DILocation(line: 18, column: 3, scope: !19)
+// CHECK:STDOUT: