Add support for emitting ~generic vectorized C++ code.

The compiler now supports an --emit-c++ option, which generates generic
vector C++ code.  To actually compile this code, the user must provide
C++ code that implements a variety of types and operations (e.g. adding
two floating-point vector values together, comparing them, etc).

There are two examples of this required code in examples/intrinsics:
generic-16.h is a "generic" 16-wide implementation that does all required
with scalar math; it's useful for demonstrating the requirements of the
implementation.  Then, sse4.h shows a simple implementation of a SSE4
target that maps the emitted function calls to SSE intrinsics.

When using these example implementations with the ispc test suite,
all but one or two tests pass with gcc and clang on Linux and OSX.
There are currently ~10 failures with icc on Linux, and ~50 failures with
MSVC 2010.  (To be fixed in coming days.)

Performance varies: when running the examples through the sse4.h
target, some have the same performance as when compiled with --target=sse4
from ispc directly (options), while noise is 12% slower, rt is 26%
slower, and aobench is 2.2x slower.  The details of this haven't yet been
carefully investigated, but will be in coming days as well.

Issue #92.

ispc.vcxproj

@@ -13,6 +13,7 @@
   <ItemGroup>
     <ClCompile Include="ast.cpp" />
     <ClCompile Include="builtins.cpp" />
+    <ClCompile Include="cbackend.cpp" />
     <ClCompile Include="ctx.cpp" />
     <ClCompile Include="decl.cpp" />
     <ClCompile Include="expr.cpp" />