Most callers are probably interested in
`Compilation.ScriptCompilationInfo.PreviousScriptCompilation` as well.
Behavioral changes expected in QuickInfo (fix crash) and SymbolFinder
(offer additional CodeFixes).

Fixes #6616 and #6657.

Moves the build task code to a shared project that we use to build
at net45 and a portable50 CodeAnalysis MSBuild task. Also upgrades to
nuget v3.2.1-rc since this is necessary to understand the dotnet4.3 spec
used in the project.json.

It's always supposed to be true at this point, and only half the things
were looking at it in the first place.

...to Roslyn.VisualStudio.Setup.vsix

The official build flag specifies which assembly version is baked into
Roslyn. This was being used in various ways to either include or not
include binding redirects, but we now always want binding redirects.
Also, we were passing the flag along to the compiler to figure out
which NewVersion to bake into the redirects, but setting nothing
at all conveniently does exactly what we want.

...to Roslyn.VisualStudio.Setup.vsix

We sometimes need an .exe and sometimes need a .dll, so let's just
make it explicit. We can still get the assembly name either way.

Otherwise, the interactive window won't be able to find the language
services DLLs.

In Update 1, devenv.exe.config specifies the binding redirects for
the Roslyn assemblies, in addition to us specifying them in the pkgdef.
If you're building an experimental VSIX, we need to specify a temporary
flag which will enable the shell to ignore the other redirects in
the .exe.config in favor of our pkgdef ones.

 1. Replace the commas to avoid disrupting csv files.
 2. Prepend the type names, since they don't seem to be included by
 default.

Explicitly states that the change is a workaround against a debugger bug.
Moves the state cleanup to a top level finally clause.

Any exceptions caused by the EnC project become non fatal.

Otherwise, the interactive window won't be able to find the language
services DLLs.