Move some flag-like props from GlobalContext and TargetLowering to ClFlags.

Simplifies the GlobalContext constructor so that a future
change may just set up the flags and the GlobalContext
before calling into what is currently "main".
Namely this change:
https://codereview.chromium.org/997773002/

This also moves all uses of LLVM's CommandLine.h to
a single file, so that in the future we may be able
to simplify the flags parsing (especially for the
minimal build).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4091
BUG= https://code.google.com/p/nativeclient/issues/detail?id=4084
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1024203002
diff --git a/src/IceRNG.h b/src/IceRNG.h
index 174e3e0..bf01e41 100644
--- a/src/IceRNG.h
+++ b/src/IceRNG.h
@@ -27,7 +27,7 @@
   RandomNumberGenerator &operator=(const RandomNumberGenerator &) = delete;
 
 public:
-  explicit RandomNumberGenerator(llvm::StringRef Salt);
+  explicit RandomNumberGenerator(uint64_t Seed, llvm::StringRef Salt = "");
   uint64_t next(uint64_t Max);
 
 private: