Add (false positive) removals to ABI checker file

(cherry picked from commit 72c0006)

Author: airspeedswift

test/api-digester/Outputs/stability-stdlib-abi.swift.expected

@@ -49,3 +49,6 @@ Protocol SIMD has added inherited protocol Decodable
 Protocol SIMD has added inherited protocol Encodable
 Protocol SIMD has generic signature change from <τ_0_0 : CustomStringConvertible, τ_0_0 : ExpressibleByArrayLiteral, τ_0_0 : Hashable, τ_0_0 : SIMDStorage, τ_0_0.MaskStorage : SIMD, τ_0_0.MaskStorage.Scalar : FixedWidthInteger, τ_0_0.MaskStorage.Scalar : SignedInteger> to <τ_0_0 : CustomStringConvertible, τ_0_0 : Decodable, τ_0_0 : Encodable, τ_0_0 : ExpressibleByArrayLiteral, τ_0_0 : Hashable, τ_0_0 : SIMDStorage, τ_0_0.MaskStorage : SIMD, τ_0_0.MaskStorage.Scalar : FixedWidthInteger, τ_0_0.MaskStorage.Scalar : SignedInteger>
 Protocol SIMDStorage has generic signature change from <τ_0_0.Scalar : Hashable> to <τ_0_0.Scalar : Decodable, τ_0_0.Scalar : Encodable, τ_0_0.Scalar : Hashable>
+Func Sequence.flatMap(_:) has been removed
+Subscript String.UnicodeScalarView.subscript(_:) has been removed
+Subscript Substring.subscript(_:) has been removed