Move function_ test files to be in function dir and remove the function_ prefix

Follow up of carbon-language#5645 (comment) and carbon-language#5607.

Author: bricknerb

toolchain/check/testdata/interop/cpp/function_full_semir.carbon renamed to toolchain/check/testdata/interop/cpp/function/full_semir.carbon

@@ -7,9 +7,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_full_semir.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/full_semir.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_full_semir.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/full_semir.carbon
 
 // ============================================================================
 // Function with short parameter

toolchain/check/testdata/interop/cpp/function_param_int16.carbon renamed to toolchain/check/testdata/interop/cpp/function/param_int16.carbon

@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_param_int16.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/param_int16.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_param_int16.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/param_int16.carbon
 
 // ============================================================================
 // short

toolchain/check/testdata/interop/cpp/function_param_int32.carbon renamed to toolchain/check/testdata/interop/cpp/function/param_int32.carbon

@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_param_int32.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/param_int32.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_param_int32.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/param_int32.carbon
 
 // ============================================================================
 // int

toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon renamed to toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon

@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_param_unsupported.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/param_unsupported.carbon
 
 // ============================================================================
 // Unsupported primitive type

toolchain/check/testdata/interop/cpp/function_return.carbon renamed to toolchain/check/testdata/interop/cpp/function/return.carbon

@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function_return.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/return.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function_return.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/return.carbon
 
 // ============================================================================
 // short