Remove concrete floating-point inits from integer types (#19624)

Also removes .gyb from FloatingPoint.swift, since we barely are using it after this change.

Rework Self._convert from integer, by making it require RawSignificand: FixedWidthInteger. This requirement should have always been there, and the existing implementaiton wouldn't have actually worked correctly without it. Making it explcit makes the implementation quite a bit simpler, which is nice.

Additionally add a  fast-path conversion that will catch all concrete integer types without needing to be a concrete implementation itself. Room for further improvement, but good start. See RandomDouble performance improvements, for example.

Author: stephentyrone

stdlib/public/SDK/CoreGraphics/CGFloat.swift.gyb

@@ -56,13 +56,6 @@ public struct CGFloat {
     self.native = value.native
   }
 
-% for src_ty in all_integer_types(word_bits):
-  @_transparent public init(_ value: ${src_ty.stdlib_name}) {
-    self.native = NativeType(value)
-  }
-
-% end
-
   /// The native value.
   public var native: NativeType
 }

stdlib/public/core/CMakeLists.txt

@@ -66,7 +66,7 @@ set(SWIFTLIB_ESSENTIAL
   FixedArray.swift.gyb
   FlatMap.swift
   Flatten.swift
-  FloatingPoint.swift.gyb
+  FloatingPoint.swift
   FloatingPointParsing.swift.gyb
   FloatingPointTypes.swift.gyb
   Hashable.swift