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/ShareModuleScopeVariables.cpp b/lib/ShareModuleScopeVariables.cpp
index afbb5a3..61f754c 100644
--- a/lib/ShareModuleScopeVariables.cpp
+++ b/lib/ShareModuleScopeVariables.cpp
@@ -22,10 +22,11 @@
 #include "llvm/Pass.h"
 #include "llvm/Support/raw_ostream.h"
 
-#include "ArgKind.h"
 #include "clspv/AddressSpace.h"
 #include "clspv/Option.h"
-#include "clspv/Passes.h"
+
+#include "ArgKind.h"
+#include "Passes.h"
 
 using namespace llvm;
 
@@ -67,12 +68,12 @@
   EntryPointMap function_to_entry_points_;
 };
 
-char ShareModuleScopeVariablesPass::ID = 0;
-static RegisterPass<ShareModuleScopeVariablesPass>
-    X("ShareModuleScopeVariablesPass", "Share module scope variables");
-
 } // namespace
 
+char ShareModuleScopeVariablesPass::ID = 0;
+INITIALIZE_PASS(ShareModuleScopeVariablesPass, "ShareModuleScopeVariablesPass",
+                "Share module scope variables", false, false)
+
 namespace clspv {
 ModulePass *createShareModuleScopeVariablesPass() {
   return new ShareModuleScopeVariablesPass();