Compiler: Cygwin is identified by os(Cygwin) instead of os(Windows)

Since MinGW, Cygwin and MSVC are definitely vary greatly in ABI and C language
level behavior, it is neccessary to distinguish each other in Swift. (In target
name, MSVC is x86_64-*-windows-msvc, Cygwin is x86_64-*-windows-cygnus)

Before change, os(Windows) was *-*-windows-*, and now,
os(Cygwin) is for *-*-windows-cygnus, and os(Windows) is for the rest.

Author: tinysun212

lib/Basic/LangOptions.cpp

@@ -33,6 +33,7 @@ static const StringRef SupportedConditionalCompilationOSs[] = {
   "Windows",
   "Android",
   "PS4",
+  "Cygwin"
 };
 
 static const StringRef SupportedConditionalCompilationArches[] = {
@@ -125,6 +126,8 @@ std::pair<bool, bool> LangOptions::setTarget(llvm::Triple triple) {
     addPlatformConditionValue("os", "Linux");
   else if (triple.isOSFreeBSD())
     addPlatformConditionValue("os", "FreeBSD");
+  else if (triple.isWindowsCygwinEnvironment())
+    addPlatformConditionValue("os", "Cygwin");
   else if (triple.isOSWindows())
     addPlatformConditionValue("os", "Windows");
   else if (triple.isPS4())

test/Parse/ConditionalCompilation/x64CygwinTarget.swift

@@ -0,0 +1,8 @@
+// RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-windows-cygnus -disable-objc-interop -D FOO -parse-stdlib
+// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-cygnus
+
+#if arch(x86_64) && os(Cygwin) && _runtime(_Native)
+class C {}
+var x = C()
+#endif
+var y = x

test/Parse/ConditionalCompilation/x64WindowsTarget.swift

@@ -1,5 +1,5 @@
-// RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-windows-cygnus -disable-objc-interop -D FOO -parse-stdlib
-// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-cygnus
+// RUN: %swift -parse %s -verify -D FOO -D BAR -target x86_64-unknown-windows-msvc -disable-objc-interop -D FOO -parse-stdlib
+// RUN: %swift-ide-test -test-input-complete -source-filename=%s -target x86_64-unknown-windows-msvc
 
 #if arch(x86_64) && os(Windows) && _runtime(_Native) && _endian(little)
 class C {}