Make the source map available as filecheck variables.

This makes it possible to refer to entities defined in the source file,
using the source names prefixed with $.

For example, $v20 refers to the value by that name in the sources, even
if it was renumbered to 'vx0' in the parsed file.

filetests/parser/rewrite.cton

@@ -16,9 +16,9 @@ ebb100(v20: i32):
     trap
 }
 ; sameln: function defs() {
-; nextln: ebb0(vx0: i32):
-; nextln:     v0 = iconst.i32x8 5
-; nextln:     v1 = f64const 0x1.0000000000000p2
+; nextln: $ebb100($v20: i32):
+; nextln:     $v1000 = iconst.i32x8 5
+; nextln:     $vx200 = f64const 0x1.0000000000000p2
 ; nextln:     trap
 ; nextln: }