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/Compiler.cpp b/lib/Compiler.cpp
index 731727e..7eb8339 100644
--- a/lib/Compiler.cpp
+++ b/lib/Compiler.cpp
@@ -37,6 +37,7 @@
 #include "clspv/opencl_builtins_header.h"
 
 #include "FrontendPlugin.h"
+#include "Passes.h"
 
 #include <cassert>
 #include <numeric>