[CodeGen][NFC] Move test builtin_signbit.cpp to CodeGen (#127814)

Because it is one of the three "misplaced" test files that appeared
under `/clang/test/Analysis` but were unrelated to the clang static
analyzer. For background see the following discourse thread:
https://discourse.llvm.org/t/taking-ownership-of-clang-test-analysis/84689/2

I placed this file in `clang/test/CodeGen` because as far as I see it
tests the code generated from `__builtin_signbit` (and there are many
other `builtin_*` test files in that directory); but I'm not familiar
with that area, so I'm not certain that it's the best place.

Author: NagyDonat