[UTC] Enable --function-signature by default

This patch enables --function-signature by default under --version 2
and makes --version 2 the default. This means that all newly created
tests will check the function signature, while leaving old tests alone.

There's two motivations for this change:

* Without --function-signature, the generated check lines may fail
  in a very hard to understand way if the test both includes a
  function definition and a call to that function. (Though we could
  address this by making the CHECK-LABEL stricter, without checking
  the full signature.)
* This actually checks that uses of the arguments in the function
  body use the correct argument, instead of matching against any
  variable.

This is a replacement for D139006 and D140212 based on the
--version mechanism.

I did not include an opt-out flag --no-function-signature because
I'm not sure we need it. Would be happy to include it though,
if desired.

Differential Revision: https://reviews.llvm.org/D145149