[lldb][lldbp-dap] On Windoows, silence warnings when building with MSVC

Fixes:
```
[6373/7138] Building CXX object tools\lldb\tools\lldb-dap\CMakeFiles\lldb-dap.dir\DAP.cpp.obj
C:\git\llvm-project\lldb\tools\lldb-dap\DAP.cpp(725) : warning C4715: '`lldb_dap::DAP::HandleObject'::`30'::<lambda_2>::operator()': not all control paths return a value
[6421/7138] Building CXX object tools\lldb\tools\lldb-dap\CMakeFiles\lldb-dap.dir\Protocol\ProtocolTypes.cpp.obj
C:\git\llvm-project\lldb\tools\lldb-dap\Protocol\ProtocolTypes.cpp(203) : warning C4715: 'lldb_dap::protocol::ToString': not all control paths return a value
C:\git\llvm-project\lldb\tools\lldb-dap\Protocol\ProtocolTypes.cpp(98) : warning C4715: 'lldb_dap::protocol::toJSON': not all control paths return a value
C:\git\llvm-project\lldb\tools\lldb-dap\Protocol\ProtocolTypes.cpp(72) : warning C4715: 'lldb_dap::protocol::toJSON': not all control paths return a value
C:\git\llvm-project\lldb\tools\lldb-dap\Protocol\ProtocolTypes.cpp(111) : warning C4715: 'lldb_dap::protocol::toJSON': not all control paths return a value
[6426/7138] Building CXX object tools\lldb\tools\lldb-dap\CMakeFiles\lldb-dap.dir\Protocol\ProtocolBase.cpp.obj
C:\git\llvm-project\lldb\tools\lldb-dap\Protocol\ProtocolBase.cpp(287) : warning C4715: 'lldb_dap::protocol::fromJSON': not all control paths return a value
```

Author: aganea

lldb/tools/lldb-dap/DAP.cpp

@@ -722,6 +722,7 @@ bool DAP::HandleObject(const protocol::Message &M) {
                              case protocol::eResponseMessageNotStopped:
                                return "notStopped";
                              }
+                             llvm_unreachable("unknown response message kind.");
                            }),
                        *resp->message);
       }

lldb/tools/lldb-dap/Protocol/ProtocolBase.cpp

@@ -284,6 +284,7 @@ bool fromJSON(const json::Value &Params, Message &PM, json::Path P) {
     PM = std::move(evt);
     return true;
   }
+  llvm_unreachable("unhandled message type request.");
 }
 
 json::Value toJSON(const Message &M) {

lldb/tools/lldb-dap/Protocol/ProtocolTypes.cpp

@@ -69,6 +69,7 @@ json::Value toJSON(const ColumnType &T) {
   case eColumnTypeTimestamp:
     return "unixTimestampUTC";
   }
+  llvm_unreachable("unhandled column type.");
 }
 
 json::Value toJSON(const ColumnDescriptor &CD) {
@@ -95,6 +96,7 @@ json::Value toJSON(const ChecksumAlgorithm &CA) {
   case eChecksumAlgorithmTimestamp:
     return "timestamp";
   }
+  llvm_unreachable("unhandled checksum algorithm.");
 }
 
 json::Value toJSON(const BreakpointModeApplicability &BMA) {
@@ -108,6 +110,7 @@ json::Value toJSON(const BreakpointModeApplicability &BMA) {
   case eBreakpointModeApplicabilityInstruction:
     return "instruction";
   }
+  llvm_unreachable("unhandled breakpoint mode applicability.");
 }
 
 json::Value toJSON(const BreakpointMode &BM) {
@@ -200,6 +203,7 @@ static llvm::StringLiteral ToString(AdapterFeature feature) {
   case eAdapterFeatureTerminateDebuggee:
     return "supportTerminateDebuggee";
   }
+  llvm_unreachable("unhandled adapter feature.");
 }
 
 json::Value toJSON(const Capabilities &C) {