Delete Reloc::Default.

Having an enum member named Default is quite confusing: Is it distinct
from the others?

This patch removes that member and instead uses Optional<Reloc> in
places where we have a user input that still hasn't been maped to the
default value, which is now clear has no be one of the remaining 3
options.

llvm-svn: 269988
diff --git a/llvm/unittests/MI/LiveIntervalTest.cpp b/llvm/unittests/MI/LiveIntervalTest.cpp
index a976917..dc555c1 100644
--- a/llvm/unittests/MI/LiveIntervalTest.cpp
+++ b/llvm/unittests/MI/LiveIntervalTest.cpp
@@ -45,7 +45,7 @@
 
   TargetOptions Options;
   return std::unique_ptr<TargetMachine>(
-      T->createTargetMachine("x86_64", "", "", Options, Reloc::Default,
+      T->createTargetMachine("x86_64", "", "", Options, None,
                              CodeModel::Default, CodeGenOpt::Aggressive));
 }