Use report_fatal_error for unsupported calling conventions

The calling convention can be specified by the user in IR. Failing to support 
a particular calling convention isn't a programming error, and so relying on 
llvm_unreachable to catch and report an unsupported calling convention is not 
appropriate.

Differential Revision: https://reviews.llvm.org/D36830

llvm-svn: 311435
diff --git a/llvm/lib/Target/Lanai/LanaiISelLowering.cpp b/llvm/lib/Target/Lanai/LanaiISelLowering.cpp
index 0a9cac2..f14e45c 100644
--- a/llvm/lib/Target/Lanai/LanaiISelLowering.cpp
+++ b/llvm/lib/Target/Lanai/LanaiISelLowering.cpp
@@ -405,7 +405,7 @@
   case CallingConv::Fast:
     return LowerCCCArguments(Chain, CallConv, IsVarArg, Ins, DL, DAG, InVals);
   default:
-    llvm_unreachable("Unsupported calling convention");
+    report_fatal_error("Unsupported calling convention");
   }
 }
 
@@ -431,7 +431,7 @@
     return LowerCCCCallTo(Chain, Callee, CallConv, IsVarArg, IsTailCall, Outs,
                           OutVals, Ins, DL, DAG, InVals);
   default:
-    llvm_unreachable("Unsupported calling convention");
+    report_fatal_error("Unsupported calling convention");
   }
 }