Re-apply PR swiftlang#82750: Reimplement floating-point description implementation in Swift.

This reverts PR swiftlang#84576, which was a revert of PR swiftlang#82750
It reverts commit 4ac18aa, reversing
changes made to b46eddb.

Author: tbkka

Runtimes/Core/core/CMakeLists.txt

@@ -80,6 +80,7 @@ add_library(swiftCore
   FlatMap.swift
   Flatten.swift
   FloatingPoint.swift
+  FloatingPointToString.swift
   Hashable.swift
   AnyHashable.swift # ORDER DEPENDENCY
   Hasher.swift

Runtimes/Core/runtime/CMakeLists.txt

@@ -52,7 +52,6 @@ add_library(swiftRuntime OBJECT
     RefCount.cpp
     ReflectionMirror.cpp
     RuntimeInvocationsTracking.cpp
-    SwiftDtoa.cpp
     SwiftTLSContext.cpp
     ThreadingError.cpp
     Tracing.cpp

include/swift/Runtime/SwiftDtoa.h

@@ -261,6 +261,7 @@ split_embedded_sources(
 
     NORMAL AtomicInt.swift.gyb
   EMBEDDED FloatingPointParsing.swift.gyb
+  EMBEDDED FloatingPointToString.swift
   EMBEDDED FloatingPointTypes.swift.gyb
   EMBEDDED IntegerTypes.swift.gyb
   EMBEDDED LegacyInt128.swift.gyb