Add llvm_unreachable after fully-covered switches to appease GCC

llvm-svn: 204318
diff --git a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFObjectWriter.cpp b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFObjectWriter.cpp
index 1e3d4b7..ba127c6 100644
--- a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFObjectWriter.cpp
+++ b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFObjectWriter.cpp
@@ -74,6 +74,7 @@
   case PPCMCExpr::VK_PPC_HIGHESTA:
     return MCSymbolRefExpr::VK_PPC_HIGHESTA;
   }
+  llvm_unreachable("unknown PPCMCExpr kind");
 }
 
 unsigned PPCELFObjectWriter::getRelocTypeInner(const MCValue &Target,