Give files shorthand names in textual SemIR (#8104)

Shorthand names are intended to be used in the textual format of
references to instructions in another IR (which can't happen now, but
will be possible in a forthcoming PR). The shorthand name uses the file
name (minus the `.carbon` suffix) as a base, and if that base name is
ambiguous within the compilation, it is given a numerical suffix `.N`,
just like other textual IR names. Shorthand names are prefixed with `$`
in the textual format, to distinguish them from inst and scope names.

For example, a file `foo/bar.carbon` has the shorthand name `$bar`, and
an instruction `@baz.%qux` in that file can be referenced from another
file as `$bar.@baz.%qux`.

This PR adds `toolchain/check/testdata/basics/file_name.carbon` to test
this behavior. All other test changes are just applying this change to
the existing textual SemIR dumps.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/check/testdata/interop/cpp/function/export/generic.carbon

@@ -185,7 +185,7 @@ void CallUnsupportedF() {
 }
 ''';
 
-// CHECK:STDOUT: --- generic_facet.carbon
+// CHECK:STDOUT: --- generic_facet.carbon = $generic_facet
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   type: type = facet_type <type> [concrete]
@@ -528,7 +528,7 @@ void CallUnsupportedF() {
 // CHECK:STDOUT:   %C.val => constants.%C.val.b0d
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- generic_type_impls_interface.carbon
+// CHECK:STDOUT: --- generic_type_impls_interface.carbon = $generic_type_impls_interface
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   type: type = facet_type <type> [concrete]