[libTooling] Add Stencil constructor.

[ymandel]

Summary

Adds a constructor that takes a vector with which to initialize the Parts
field and a corresponding free function that forwards to the constructor. These
definitions are needed to assist in transitioning away from Stencil as a class
to defining it as a type alias.

[ilya-biryukov]

LGTM.

[ymandel]

renamed free function to avoid overloading a templated function.

clang/include/clang/Tooling/Transformer/Stencil.h

 };
 
 /// A sequence of code fragments, references to parameters and code-generation
 /// operations that together can be evaluated to (a fragment of) source code,
 /// given a match result.
 class Stencil {
 public:
   Stencil() = default;
+  Stencil(std::vector<StencilPart> Parts) : Parts(std::move(Parts)) {}
 
   /// Composes a stencil from a series of parts.
   template <typename... Ts> static Stencil cat(Ts &&... Parts) {
     Stencil S;
     S.Parts = {wrap(std::forward<Ts>(Parts))...};
     return S;
   }
 

 //
 // Functions for conveniently building stencils.
 //
 
 /// Convenience wrapper for Stencil::cat that can be imported with a using decl.
 template <typename... Ts> Stencil cat(Ts &&... Parts) {
   return Stencil::cat(std::forward<Ts>(Parts)...);
 }
+/// Convenience wrapper for Stencil constructor of the same type. Declaration
+/// outside of the class supports transition of `Stencil` type to an alias
+/// rather than a class.
+inline Stencil catVector(std::vector<StencilPart> Parts) {
+  return Stencil(std::move(Parts));
+}
 
 /// \returns exactly the text provided.
 StencilPart text(llvm::StringRef Text);
 
 /// \returns the source corresponding to the selected range.
 StencilPart selection(RangeSelector Selector);
 
 /// Generates the source of the expression bound to \p Id, wrapping it in