Add an API to construct an XcodeSDK from an SDK type.

[aprantl]

Summary

Unfortunately I discovered that I do need to be able to go full round-trip in swift-lldb.

Also, this moves numSDKs out of the actual enum, as to not mess with the switch-cases-covered warning.

[vsk]

lldb/source/Utility/XcodeSDK.cpp

44

I'd expect -Wcovered-switch to break the build if a new case gets added and this list isn't updated. Wdyt of dropping numSDKTypes and using an llvm_unreachable here instead?

[aprantl]

Remove redundant static_assert. It was only needed in the other function because that one used cascading ifs.

[aprantl]

(Upload the full patch)

[JDevlieghere]

lldb/source/Utility/XcodeSDK.cpp

145

Cool :P

lldb/source/Utility/XcodeSDK.cpp

case XcodeSDK::watchOS:
     return "WatchOS";
   case XcodeSDK::bridgeOS:
     return "bridgeOS";
   case XcodeSDK::Linux:
     return "Linux";
   case XcodeSDK::unknown:
     return {};
   }
-  static_assert(XcodeSDK::Linux == XcodeSDK::numSDKTypes - 1,
-                "New SDK type was added, update this list!");
 }
 
 XcodeSDK::XcodeSDK(XcodeSDK::Info info) : m_name(GetName(info.type).str()) {
   if (!m_name.empty()) {
     if (!info.version.empty())
       m_name += info.version.getAsString();
     if (info.internal)
       m_name += ".Internal";
     m_name += ".sdk";

 llvm::StringRef XcodeSDK::GetString() const { return m_name; }
 
 bool XcodeSDK::Info::operator<(const Info &other) const {
   return std::tie(type, version, internal) <
          std::tie(other.type, other.version, other.internal);
 }
 
 bool XcodeSDK::Info::operator==(const Info &other) const {
   return std::tie(type, version, internal) ==
          std::tie(other.type, other.version, other.internal);
 }
 
 void XcodeSDK::Merge(XcodeSDK other) {
   // The "bigger" SDK always wins.
   auto l = Parse();
   auto r = other.Parse();
   if (l < r)