[AutoDiff] SILGen differentiability witnesses.

[dan-zheng]

Generate SIL differentiability witnesses from AST @differentiable attributes,
using lowered parameter indices, result indices (currently with capacity 1 and
set index 0), and derivative generic signature.

Resolves TF-869.

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The TF-866 master issue tracks all retroactive derivative registration tasks.

Todos related to this patch:

• TF-835: lower @differentiating attribute to SIL [differentiable] attribute instead of creating implicit @differentiable attributes. This is blocked by full retroactive derivative registration, see issue comment for details.
• TF-919: explore emitting serialized differentiability witnesses. Currently, SILGen never creates serialized differentiability witnesses to avoid deserialization issues.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

CI fails due to differentiability witness deserialization failures (see occurrences of UNKNOWN):

  Block ID #14 (SIL_BLOCK):
      Num Instances: 1
         Total Size: 12694b/1586.75B/396W
    Percent of file: 4.9487%
      Num SubBlocks: 0
        Num Abbrevs: 46
        Num Records: 56
    Percent Abbrevs: 100.0000%

	Record Histogram:
		  Count    # Bits     b/Rec   % Abv  Record Kind
		     16      3040     190.0  100.00  SIL_FUNCTION
		     10      2221     222.1  100.00  UnknownCode35
		      8       640      80.0  100.00  SIL_ONE_OPERAND
		      6      1144     190.7  100.00  SIL_INST_APPLY
		      4       452     113.0  100.00  SIL_ONE_TYPE_ONE_OPERAND
		      4       196      49.0  100.00  SIL_ONE_TYPE
		      4       792     198.0  100.00  SIL_BASIC_BLOCK
		      2       198      99.0  100.00  decls_block::PROTOCOL_CONFORMANCE_XREF
		      2       534     267.0  100.00  SIL_INST_WITNESS_METHOD

  Block ID #15 (SIL_INDEX_BLOCK):
      Num Instances: 1
         Total Size: 5686b/710.75B/177W
    Percent of file: 2.2167%
      Num SubBlocks: 0
        Num Abbrevs: 2
        Num Records: 4
    Percent Abbrevs: 100.0000%

	Record Histogram:
		  Count    # Bits     b/Rec   % Abv  Record Kind
		      1       324            100.00  SIL_DIFFERENTIABILITY_WITNESS_OFFSETS
		      1      1762            100.00  UnknownCode12
		      1       510            100.00  SIL_FUNC_OFFSETS
		      1      2936            100.00  SIL_FUNC_NAMES

These surfaced only after differentiability witness SILGen.

EDIT: Fixed in 4c4bc43.

[dan-zheng]

@swift-ci Please test tensorflow