Platform: alias math constants on Windows

The non-C/C++ standard defined math constants require
`-Xcc -D_USE_MATH_DEFINES` when using the `ucrt` module.  Instead,
provide them through the wrapper `MSVCRT` module as aliases so that the
user does not need to be aware of this in practice.

Repairs the Windows build of `_Differentiation`.

Author: compnerd

stdlib/public/Platform/msvcrt.swift

@@ -84,3 +84,6 @@ public let DBL_MIN = Double.leastNormalMagnitude
 @available(swift, deprecated: 3.0, message: "Please use 'Double.leastNonzeroMagnitude' or '.leastNonzeroMagnitude'.")
 public let DBL_TRUE_MIN = Double.leastNonzeroMagnitude
 
+public let M_LN2 = ucrt.M_LN2
+public let M_LOG10E = ucrt.M_LOG10E
+public let M_2_SQRTPI = ucrt.M_2_SQRTPI