It's a CPP macro, documented in the source code for reflection:

-- Due to https://gitlab.haskell.org/ghc/ghc/issues/16893, inlining
-- unsafeCoerce too aggressively can cause optimization to become unsound on
-- old versions of GHC. As a workaround, we mark unsafeCoerce-using definitions
-- as NOINLINE where necessary.
-- See https://github.com/ekmett/reflection/issues/47.
#if __GLASGOW_HASKELL__ >= 811
# define INLINE_UNSAFE_COERCE INLINE
#else
# define INLINE_UNSAFE_COERCE NOINLINE
#endif