Fix TBDGen. Remove `@inlinable` from `func differentiableFunction(from:)`.
Use lowered parameter indices rather than AST parameter indices for SIL diff witness TBDGen.
There are only two remaining errors during TensorFlow library compilation:
```
<unknown>:0: error: symbol 'AD__$s10TensorFlow13SimpleRNNCellV5StateVyAEyx_GAA0A0VyxGcfC_PSURS10TensorFlow0aB13FloatingPointRzl' (AD__$s10TensorFlow13SimpleRNNCellV5StateVyAEyx_GAA0A0VyxGcfC_PSURS10TensorFlow0aB13FloatingPointRzl) is in generated IR file, but not in TBD file
<unknown>:0: error: symbol 'AD__$ss15_DifferentiableP10TensorFlowE9sequenced7through_____6OutputQyd_4_qd___qd_0_qd_1_qd_2_qd_3_qd_4_t5InputQyd__RszAC5LayerRd__AcJRd_0_AcJRd_1_AcJRd_2_AcJRd_3_AcJRd_4_AHQyd_0_AFRtd__AHQyd_1_AFRtd_0_AHQyd_2_AFRtd_1_AHQyd_3_AFRtd_2_AHQyd_4_AFRtd_3_r4_lF_PSSSSSSSRS' (AD__$ss15_DifferentiableP10TensorFlowE9sequenced7through_____6OutputQyd_4_qd___qd_0_qd_1_qd_2_qd_3_qd_4_t5InputQyd__RszAC5LayerRd__AcJRd_0_AcJRd_1_AcJRd_2_AcJRd_3_AcJRd_4_AHQyd_0_AFRtd__AHQyd_1_AFRtd_0_AHQyd_2_AFRtd_1_AHQyd_3_AFRtd_2_AHQyd_4_AFRtd_3_r4_lF_PSSSSSSSRS) is in generated IR file, but not in TBD file
```
After removing `@inlinable` from `func differentiableFunction(from:)`, there are
only 8 failing AutoDiff tests (some of which may be easy to fix):
```
Failing Tests (8):
Swift(macosx-x86_64) :: AutoDiff/closures.swift
Swift(macosx-x86_64) :: AutoDiff/differentiable_sil_attr_roundtrip.swift
Swift(macosx-x86_64) :: AutoDiff/forward_mode_sil.swift
Swift(macosx-x86_64) :: AutoDiff/refcounting.swift
Swift(macosx-x86_64) :: AutoDiff/silgen_thunking/main.swift
Swift(macosx-x86_64) :: AutoDiff/subset_parameters_thunk.swift
Swift(macosx-x86_64) :: AutoDiff/tbdgen.swift
Swift(macosx-x86_64) :: AutoDiff/witness_table_sil.swift
```
swift
d238abc4 - Fix TBDGen. Remove `@inlinable` from `func differentiableFunction(from:)`.
