Add a distinct semir file separator. (#3438)

This removes the filename from the file-scoped block, and places it
above to make it clear where the full SemIR begins (with multifile,
providing a barrier between).

toolchain/check/testdata/if_expr/basic.carbon

@@ -8,7 +8,9 @@ fn F(b: bool, n: i32, m: i32) -> i32 {
   return if b then n + m else m + n;
 }
 
-// CHECK:STDOUT: file "basic.carbon" {
+// CHECK:STDOUT: --- basic.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -33,3 +35,4 @@ fn F(b: bool, n: i32, m: i32) -> i32 {
 // CHECK:STDOUT:   %.loc8_10: i32 = block_arg !if.expr.result
 // CHECK:STDOUT:   return %.loc8_10
 // CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/if_expr/constant_condition.carbon

@@ -15,7 +15,9 @@ fn G() -> i32 {
   return if false then A() else B();
 }
 
-// CHECK:STDOUT: file "constant_condition.carbon" {
+// CHECK:STDOUT: --- constant_condition.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
 // CHECK:STDOUT:   %A: <function> = fn_decl @A
 // CHECK:STDOUT:   %B: <function> = fn_decl @B
 // CHECK:STDOUT:   %F: <function> = fn_decl @F
@@ -81,3 +83,4 @@ fn G() -> i32 {
 // CHECK:STDOUT:   %.loc15_10: i32 = block_arg !if.expr.result
 // CHECK:STDOUT:   return %.loc15_10
 // CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/if_expr/control_flow.carbon

@@ -11,7 +11,9 @@ fn F(b: bool) -> i32 {
   return if b then A() else B();
 }
 
-// CHECK:STDOUT: file "control_flow.carbon" {
+// CHECK:STDOUT: --- control_flow.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
 // CHECK:STDOUT:   %A: <function> = fn_decl @A
 // CHECK:STDOUT:   %B: <function> = fn_decl @B
 // CHECK:STDOUT:   %F: <function> = fn_decl @F
@@ -52,3 +54,4 @@ fn F(b: bool) -> i32 {
 // CHECK:STDOUT:   %.loc11_10: i32 = block_arg !if.expr.result
 // CHECK:STDOUT:   return %.loc11_10
 // CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/if_expr/fail_not_in_function.carbon

@@ -33,11 +33,13 @@ class C {
   var n: if true then i32 else f64;
 }
 
+// CHECK:STDOUT: --- fail_not_in_function.carbon
+// CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.loc34: type = struct_type {.n: <error>}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: file "fail_not_in_function.carbon" {
+// CHECK:STDOUT: file {
 // CHECK:STDOUT:   %.loc17: i32 = block_arg <unexpected instblockref block4>
 // CHECK:STDOUT:   %x: i32 = bind_name x, %.loc17
 // CHECK:STDOUT:   class_decl @C, ()
@@ -51,3 +53,4 @@ class C {
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .n = <unexpected instref inst+20>
 // CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/if_expr/nested.carbon

@@ -8,7 +8,9 @@ fn F(a: bool, b: bool, c: bool) -> i32 {
   return if a then if b then 1 else 2 else if c then 3 else 4;
 }
 
-// CHECK:STDOUT: file "nested.carbon" {
+// CHECK:STDOUT: --- nested.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -53,3 +55,4 @@ fn F(a: bool, b: bool, c: bool) -> i32 {
 // CHECK:STDOUT:   %.loc8_10: i32 = block_arg !if.expr.result.loc8_10
 // CHECK:STDOUT:   return %.loc8_10
 // CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/if_expr/struct.carbon

@@ -11,12 +11,14 @@ fn F(cond: bool) {
   G(if cond then a else a);
 }
 
+// CHECK:STDOUT: --- struct.carbon
+// CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.loc7: type = ptr_type {.a: i32, .b: i32}
 // CHECK:STDOUT:   %.loc11: type = tuple_type ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: file "struct.carbon" {
+// CHECK:STDOUT: file {
 // CHECK:STDOUT:   %G: <function> = fn_decl @G
 // CHECK:STDOUT:   %F: <function> = fn_decl @F
 // CHECK:STDOUT: }
@@ -67,3 +69,4 @@ fn F(cond: bool) {
 // CHECK:STDOUT:   %.loc11_4: init () = call %G.ref(%.loc11_5)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
+// CHECK:STDOUT: