[Hexagon] Avoid creating an empty target feature

If the CPU string is empty, the target feature map may end up having
an empty string inserted to it. The symptom of the problem is a warning
message:
  '+' is not a recognized feature for this target (ignoring feature)
Also, the target-features attribute in the module will have an empty
string in it.

clang/lib/Basic/Targets/Hexagon.cpp

@@ -98,7 +98,8 @@ bool HexagonTargetInfo::initFeatureMap(
   StringRef CPUFeature = CPU;
   CPUFeature.consume_front("hexagon");
   CPUFeature.consume_back("t");
-  Features[CPUFeature] = true;
+  if (!CPUFeature.empty())
+    Features[CPUFeature] = true;
 
   Features["long-calls"] = false;
 

clang/test/CodeGen/hexagon-empty-cpu-feature.c

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 %s -triple hexagon-unknown-elf -emit-llvm -o - 2>&1 | FileCheck %s
+// CHECK-NOT: '+' is not a recognized feature for this target
+
+// Empty