Adds accessor methods to class ClFlags.

Allows one to define explicit overrides in get accessors, based on
compilation features. To show usage, modified SubConstantCalls to
never be enabled if building a minimal llvm2ice.

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/905463003
diff --git a/unittest/BitcodeMunge.cpp b/unittest/BitcodeMunge.cpp
index ec57ffa..623e3be 100644
--- a/unittest/BitcodeMunge.cpp
+++ b/unittest/BitcodeMunge.cpp
@@ -26,8 +26,8 @@
   setupTest(TestName, Munges, MungeSize, AddHeader);
 
   Ice::ClFlags Flags;
-  Flags.AllowErrorRecovery = true;
-  Flags.GenerateUnitTestMessages = true;
+  Flags.setAllowErrorRecovery(true);
+  Flags.setGenerateUnitTestMessages(true);
   Ice::GlobalContext Ctx(DumpStream, DumpStream, nullptr,
                          Ice::IceV_Instructions, Ice::Target_X8632, Ice::Opt_m1,
                          "", Flags);