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/function/call/struct_param.carbon

@@ -16,6 +16,7 @@ fn Main() {
   F({}, {.a = 1}, {.a = 2, .b = 3});
 }
 
+// CHECK:STDOUT: ; ---
 // CHECK:STDOUT: ; ModuleID = 'struct_param.carbon'
 // CHECK:STDOUT: source_filename = "struct_param.carbon"
 // CHECK:STDOUT:
@@ -56,3 +57,4 @@ fn Main() {
 // CHECK:STDOUT: !14 = !{null}
 // CHECK:STDOUT: !15 = !DILocation(line: 16, column: 3, scope: !12)
 // CHECK:STDOUT: !16 = !DILocation(line: 15, column: 1, scope: !12)
+// CHECK:STDOUT: