Fix 'control reaches end of non-void' warning by using llvm_unreachable.

llvm-svn: 319318
diff --git a/clang/lib/Format/BreakableToken.cpp b/clang/lib/Format/BreakableToken.cpp
index db350b9..a5fabfe 100644
--- a/clang/lib/Format/BreakableToken.cpp
+++ b/clang/lib/Format/BreakableToken.cpp
@@ -195,9 +195,8 @@
                                                 unsigned Offset,
                                                 StringRef::size_type Length,
                                                 unsigned StartColumn) const {
-  assert(false &&
-         "Getting the length of a part of the string literal indicates that "
-         "the code tries to reflow it.");
+  llvm_unreachable("Getting the length of a part of the string literal "
+                   "indicates that the code tries to reflow it.");
 }
 
 unsigned