[LLVM][TargetParser] Handle -msys targets the same as -cygwin.

MSYS2 uses i686-pc-msys and x86_64-pc-msys as target, and is a fork of
Cygwin.  There's an effort underway to try to switch as much as possible
to use -pc-cygwin targets, but the -msys target will be hanging around
for the forseeable future.

Author: jeremyd2019

llvm/lib/TargetParser/Triple.cpp

@@ -1150,7 +1150,8 @@ std::string Triple::normalize(StringRef Str, CanonicalForm Form) {
   OSType OS = UnknownOS;
   if (Components.size() > 2) {
     OS = parseOS(Components[2]);
-    IsCygwin = Components[2].starts_with("cygwin");
+    IsCygwin = Components[2].starts_with("cygwin") ||
+               Components[2].starts_with("msys");
     IsMinGW32 = Components[2].starts_with("mingw");
   }
   EnvironmentType Environment = UnknownEnvironment;
@@ -1195,7 +1196,7 @@ std::string Triple::normalize(StringRef Str, CanonicalForm Form) {
         break;
       case 2:
         OS = parseOS(Comp);
-        IsCygwin = Comp.starts_with("cygwin");
+        IsCygwin = Comp.starts_with("cygwin") || Comp.starts_with("msys");
         IsMinGW32 = Comp.starts_with("mingw");
         Valid = OS != UnknownOS || IsCygwin || IsMinGW32;
         break;