[stdlib] convert withUnsafeMutableBytes() to typed throws

Author: glessard

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -1207,11 +1207,23 @@ extension ${Self} {
 ///     of the closure's execution.
 /// - Returns: The return value, if any, of the `body` closure.
 @inlinable
-public func withUnsafeMutableBytes<T, Result>(
+@_alwaysEmitIntoClient
+public func withUnsafeMutableBytes<T, E: Error, Result>(
+  of value: inout T,
+  _ body: (UnsafeMutableRawBufferPointer) throws(E) -> Result
+) throws(E) -> Result {
+  let pointer = UnsafeMutableRawPointer(Builtin.addressof(&value))
+  return try body(.init(start: pointer, count: MemoryLayout<T>.size))
+}
+
+/// ABI: Historical withUnsafeMutableBytes(of:_:) rethrows,
+/// expressed as "throws", which is ABI-compatible with "rethrows".
+@_silgen_name("$ss22withUnsafeMutableBytes2of_q_xz_q_SwKXEtKr0_lF")
+@usableFromInline
+func __abi_se0413_withUnsafeMutableBytes<T, Result>(
   of value: inout T,
   _ body: (UnsafeMutableRawBufferPointer) throws -> Result
-) rethrows -> Result
-{
+) throws -> Result {
   return try withUnsafeMutablePointer(to: &value) {
     return try body(UnsafeMutableRawBufferPointer(
         start: $0, count: MemoryLayout<T>.size))