Accelerate deprecate split complex inits (#29221)

* Update FFT Tests

I removed SinglePrecisionComplexConversions and DoublePrecisionComplexConversions - they weren't testing any new API.

2DSinglePrecision, 2DDoublePrecision, 1DSinglePrecision, and 1DDoublePrecision have all been updated to no longer use the deprecated initializer and avoid the temporary pointer warnings.

* Deprecate Split Complex inits

Deprecate init(fromInputArray:realParts:imaginaryParts:) for DSPSplitComplex and DSPDoubleSplitComplex

* Fix Availability

Author: FlexMonkey

stdlib/public/Darwin/Accelerate/vDSP_FFT.swift

@@ -329,8 +329,11 @@ struct vDSP_FFTFunctions {
     }
 }
 
-
-@available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+@available(macOS, introduced: 10.15, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(iOS, introduced: 13.0, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(watchOS, introduced: 6.0, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(tvOS, introduced: 13.0, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(*, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
 extension DSPSplitComplex {
 
     /// Creates a new `DSPSplitComplex` structure from a real array not in even-odd split configuration.
@@ -353,7 +356,11 @@ extension DSPSplitComplex {
     }
 }
 
-@available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
+@available(macOS, introduced: 10.15, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(iOS, introduced: 13.0, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(watchOS, introduced: 6.0, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(tvOS, introduced: 13.0, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
+@available(*, deprecated, message: "Use the `withUnsafeMutableBufferPointer` method on the real and imaginary arrays to create `DSPSplitComplex` for a defined scope.")
 extension DSPDoubleSplitComplex {
 
     /// Creates a new `DSPDoubleSplitComplex` structure from a real array not in even-odd split configuration.