TestFile preamble comments apply to all functions.

Include the test file preamble comments when building a filecheck
instance for every function in the file.

This makes it possible to define common regex variables in the preamble
and use these definitions for all the functions.

cranelift/docs/testing.rst

@@ -150,6 +150,10 @@ use filecheck will extract comments associated with each function (or its
 entities) and scan them for filecheck directives. The test output for each
 function is then matched against the filecheck directives for that function.
 
+Comments appearing before the first function in a file apply to every function.
+This is useful for defining common regular expression variables with the
+``regex:`` directive, for example.
+
 Note that LLVM's file tests don't separate filecheck directives by their
 associated function. It verifies the concatenated output against all filecheck
 directives in the test file. LLVM's :command:`FileCheck` command has a