[builtins] Drop -fnested-functions flag from tests

This flag does nothing for clang and is apparently not supported by GCC:
aarch64-linux-gnu-gcc: error: unrecognized command-line option '-fnested-functions'; did you mean '-Wunused-function'?
It looks like it was maybe supported by Apple GCC, but that is no longer
a supported compiler.

Differential Revision: https://reviews.llvm.org/D153819