[Backtracing][Windows] Use CRT, not MSVCRT.

The correct name for the runtime library appears to be CRT, not MSVCRT.

rdar://104336548

Author: al45tair

stdlib/public/Backtracing/BacktraceFormatter.swift

@@ -20,7 +20,7 @@ import Swift
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 @_implementationOnly import Darwin
 #elseif os(Windows)
-@_implementationOnly import MSVCRT
+@_implementationOnly import CRT
 #elseif os(Linux)
 @_implementationOnly import Glibc
 #endif