[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/MCSectionXCOFF.cpp b/llvm/lib/MC/MCSectionXCOFF.cpp
index a3f79c3..d00a435 100644
--- a/llvm/lib/MC/MCSectionXCOFF.cpp
+++ b/llvm/lib/MC/MCSectionXCOFF.cpp
@@ -29,12 +29,12 @@
   }
 
   if (getKind().isBSSLocal() || getKind().isCommon()) {
-    if (getMappingClass() != XCOFF::XMC_RW &&
-        getMappingClass() != XCOFF::XMC_BS)
-      llvm_unreachable("Generated a storage-mapping class for a common/bss "
-                       "csect we don't understand how to switch to.");
-    if (getCSectType() != XCOFF::XTY_CM)
-      llvm_unreachable("wrong csect type for .bss csect");
+    assert((getMappingClass() == XCOFF::XMC_RW ||
+            getMappingClass() == XCOFF::XMC_BS) &&
+           "Generated a storage-mapping class for a common/bss csect we don't "
+           "understand how to switch to.");
+    assert(getCSectType() == XCOFF::XTY_CM &&
+           "wrong csect type for .bss csect");
     // Don't have to print a directive for switching to section for commons.
     // '.comm' and '.lcomm' directives for the variable will create the needed
     // csect.