[NFC][AIX] Use assert instead of llvm_unreachable

Addresses post-commit comments on https://reviews.llvm.org/D64825. Use
assert instead of llvm_unreachable to check if invalid csect types are being
generated. Use report_fatal_error on unimplemented XCOFF features.

Differential Revision: https://reviews.llvm.org/D64825

llvm-svn: 368720
diff --git a/llvm/lib/MC/MCXCOFFStreamer.cpp b/llvm/lib/MC/MCXCOFFStreamer.cpp
index 7d526cc..9607919 100644
--- a/llvm/lib/MC/MCXCOFFStreamer.cpp
+++ b/llvm/lib/MC/MCXCOFFStreamer.cpp
@@ -60,5 +60,5 @@
 
 void MCXCOFFStreamer::EmitXCOFFLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
                                             unsigned ByteAlign) {
-  llvm_unreachable("Not implemented yet.");
-}
\ No newline at end of file
+  report_fatal_error("Emission of local commons not implemented yet.");
+}