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/MSP430/MSP430ISelLowering.cpp b/llvm/lib/Target/MSP430/MSP430ISelLowering.cpp
index dae14fd..55e371f 100644
--- a/llvm/lib/Target/MSP430/MSP430ISelLowering.cpp
+++ b/llvm/lib/Target/MSP430/MSP430ISelLowering.cpp
@@ -546,7 +546,7 @@
 
   switch (CallConv) {
   default:
-    llvm_unreachable("Unsupported calling convention");
+    report_fatal_error("Unsupported calling convention");
   case CallingConv::C:
   case CallingConv::Fast:
     return LowerCCCArguments(Chain, CallConv, isVarArg, Ins, dl, DAG, InVals);
@@ -576,7 +576,7 @@
 
   switch (CallConv) {
   default:
-    llvm_unreachable("Unsupported calling convention");
+    report_fatal_error("Unsupported calling convention");
   case CallingConv::MSP430_BUILTIN:
   case CallingConv::Fast:
   case CallingConv::C: