Re-implement clspv-opt as a standalone driver (#346)

* Fix #307 and #322.

This re-implements clspv-opt as a standalone driver.  The initial
implementation used the plugin facility for LLVM opt, but this is
difficult to implement in Windows (#307).

The driver only supports LLVM scalar transformations and all of clspv
transformations.  To do this, passes must be declared using LLVM's
INITIALIZE_PASS macro.  Additionally, the function
initializeClspvPasses() must include a call to the initialize*()
function generated by INITIALIZE_PASS.

This change also adds tests for all the existing passes in clspv.  It
does not check for specific transformations.  It checks that the flags
are accepted by clspv-opt.

Finally, this change reduces the build time for LLVM by building no
target backends by default.
diff --git a/lib/SimplifyPointerBitcastPass.cpp b/lib/SimplifyPointerBitcastPass.cpp
index 06cdd8f..7a2dc7d 100644
--- a/lib/SimplifyPointerBitcastPass.cpp
+++ b/lib/SimplifyPointerBitcastPass.cpp
@@ -18,6 +18,8 @@
 #include "llvm/Pass.h"
 #include "llvm/Support/raw_ostream.h"
 
+#include "Passes.h"
+
 using namespace llvm;
 
 #define DEBUG_TYPE "SimplifyPointerBitcast"
@@ -37,8 +39,8 @@
 } // namespace
 
 char SimplifyPointerBitcastPass::ID = 0;
-static RegisterPass<SimplifyPointerBitcastPass>
-    X("SimplifyPointerBitcast", "Simplify Pointer Bitcast Pass");
+INITIALIZE_PASS(SimplifyPointerBitcastPass, "SimplifyPointerBitcast",
+                "Simplify Pointer Bitcast Pass", false, false)
 
 namespace clspv {
 llvm::ModulePass *createSimplifyPointerBitcastPass() {