Add a feature to explicitly include a file's SemIR (#5961)

Trying to figure out an easy way to debug semir in the prelude, #5703
removed an option to set `--exclude-dump-file-prefix` to empty. But,
this is probably an improvement over that flow... With this change, it's
possible to add `//@dump-sem-ir-file` to a specific prelude file, and
its full IR will be printed. Additionally, it becomes an option with the
default `--dump-sem-ir-ranges=only` to add `//@dump-sem-ir-file` and get
the full file's IR.

Author: jonmeow

toolchain/check/check.cpp

@@ -340,17 +340,17 @@ static auto MaybeDumpFormattedSemIR(
     return;
   }
 
-  bool has_ranges = sem_ir.parse_tree().tokens().has_dump_sem_ir_ranges();
+  const auto& tokens = sem_ir.parse_tree().tokens();
   if (options.dump_sem_ir_ranges ==
           CheckParseTreesOptions::DumpSemIRRanges::Only &&
-      !has_ranges) {
+      !tokens.has_dump_sem_ir_ranges() && !tokens.has_include_in_dumps()) {
     return;
   }
 
   bool use_dump_sem_ir_ranges =
       options.dump_sem_ir_ranges !=
           CheckParseTreesOptions::DumpSemIRRanges::Ignore &&
-      has_ranges;
+      tokens.has_dump_sem_ir_ranges();
   SemIR::Formatter formatter(&sem_ir, tree_and_subtrees_getter,
                              options.include_in_dumps, use_dump_sem_ir_ranges);
   formatter.Format();

toolchain/check/testdata/basics/dump_sem_ir_ranges.carbon

@@ -91,6 +91,12 @@ library "[[@TEST_NAME]]";
 
 fn F();
 
+// --- explicit_dump_file.carbon
+
+library "[[@TEST_NAME]]";
+
+//@include-in-dumps
+
 // CHECK:STDOUT: --- function.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -320,3 +326,19 @@ fn F();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F();
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- explicit_dump_file.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/basics/dump_sem_ir_ranges_ignore.carbon

@@ -25,6 +25,12 @@ library "[[@TEST_NAME]]";
 
 fn F();
 
+// --- explicit_dump_file.carbon
+
+library "[[@TEST_NAME]]";
+
+//@include-in-dumps
+
 // CHECK:STDOUT: --- with-range.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -57,3 +63,9 @@ fn F();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F();
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- explicit_dump_file.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/basics/dump_sem_ir_ranges_only.carbon

@@ -27,6 +27,12 @@ library "[[@TEST_NAME]]";
 
 fn F();
 
+// --- explicit_dump_file.carbon
+
+library "[[@TEST_NAME]]";
+
+//@include-in-dumps
+
 // CHECK:STDOUT: --- with-range.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -40,3 +46,9 @@ fn F();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F();
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- explicit_dump_file.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT: