[xla:ffi] Change scalar attributes ABI and annotate error handling branches with XLA_FFI_PREDICT_FALSE
[xla:ffi] Change scalar attributes ABI and annotate error handling branches with XLA_FFI_PREDICT_FALSE

In preparation for adding dense array attributes change scalar attributes ABI for consistency

Small regression in decoding attributes because of one extra branch.

name                old cpu/op   new cpu/op   delta
BM_BufferBaseArgX1  9.51ns ± 5%  9.10ns ± 5%  -4.29%  (p=0.000 n=20+18)
BM_BufferBaseArgX4  17.4ns ± 7%  16.3ns ± 7%  -6.15%  (p=0.000 n=19+20)
BM_BufferArgX1      10.8ns ±11%  11.4ns ±15%  +5.86%  (p=0.010 n=20+20)
BM_BufferArgX4      19.3ns ± 8%  18.5ns ± 4%  -3.99%  (p=0.000 n=20+20)
BM_TupleOfI32Attrs  56.5ns ± 3%  60.9ns ± 3%  +7.90%  (p=0.000 n=20+20)

BM_BufferArgX1 regression looks like noise
