stdlib: replace _ascii8() with UInt8(ascii:)

Author: gribozavr

stdlib/public/core/InputStream.swift

@@ -42,8 +42,8 @@ public func readLine(strippingNewline: Bool = true) -> String? {
     // <rdar://problem/20013999> Recognize Unicode newlines in readLine()
     //
     // Recognize only LF and CR+LF combinations for now.
-    let cr = CChar(_ascii8("\r"))
-    let lf = CChar(_ascii8("\n"))
+    let cr = CChar(UInt8(ascii: "\r"))
+    let lf = CChar(UInt8(ascii: "\n"))
     if readBytes == 1 && linePtr[0] == lf {
       return ""
     }

stdlib/public/core/UnicodeScalar.swift

@@ -267,14 +267,6 @@ extension UnicodeScalar.UTF16View : RandomAccessCollection {
   }
 }
 
-/// Returns c as a UTF8.CodeUnit.  Meant to be used as _ascii8("x").
-@warn_unused_result
-public // SPI(SwiftExperimental)
-func _ascii8(_ c: UnicodeScalar) -> UTF8.CodeUnit {
-  _sanityCheck(c.value >= 0 && c.value <= 0x7F, "not ASCII")
-  return UTF8.CodeUnit(c.value)
-}
-
 /// Returns c as a UTF16.CodeUnit.  Meant to be used as _ascii16("x").
 @warn_unused_result
 public // SPI(SwiftExperimental)