[debuginfo-tests] Add some optnone tests

Add dexter tests using the optnone attribute in various scenarios. Our users
have found optnone useful when debugging optimised code. We have these tests
downstream (and one upstream already: D89873) and we would like to contribute
them if there is any interest.

The tests are fairly self explanatory. Testing optnone with:
  * optnone-fastmath.cpp: floats and -ffast-math,
  * optnone-simple-functions: simple functions and integer arithmetic,
  * optnone-struct-and-methods: a struct with methods,
  * optnone-vectors-and-functions: templates and integer vector arithmetic.

optnone-vectors-and-functions contains two FIXMEs. The first problem is that
lldb seems to struggle with evaluating expressions with the templates used
here (example below). Perhaps this is PR42920?

  (lldb) p TypeTraits<int __attribute__((ext_vector_type(4)))>::NumElements
  error: <user expression 0>:1:1: no template named 'TypeTraits'
  TypeTraits<int __attribute__((ext_vector_type(4)))>::NumEl...