Fix Visual Studio 2017 errors caused by recent Expr.h gardening

[hughbe]

We get many errors compiling Swift with VS2017, as a result of internal stdlib changes.

C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\type_traits(416): error C2139: 'swift::Expr': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_convertible_to' (compiling source file C:\Users\hughb\Documents\GitHub\swift\swift\lib\SILGen\SILGenProfiling.cpp)
C:\Users\hughb\Documents\GitHub\swift\swift\include\swift/AST/Stmt.h(32): note: see declaration of 'swift::Expr' (compiling source file C:\Users\hughb\Documents\GitHub\swift\swift\lib\SILGen\SILGenProfiling.cpp)
C:\Users\hughb\Documents\GitHub\swift\swift\include\swift/AST/Stmt.h(773): note: see reference to class template instantiation 'std::is_convertibleswift::Expr,T' being compiled
with
[
T=swift::Expr
] (compiling source file C:\Users\hughb\Documents\GitHub\swift\swift\lib\SILGen\SILGenProfiling.cpp)
C:\Users\hughb\Documents\GitHub\swift\swift\include\swift/Basic/NullablePtr.h(38): note: while compiling class template member function 'swift::NullablePtrswift::Expr::NullablePtr(swift::NullablePtr,std::enable_if<std::is_convertible<OtherT,T>::value,swift::NullablePtr::PlaceHolder>::type)'
with
[
T=swift::Expr
] (compiling source file C:\Users\hughb\Documents\GitHub\swift\swift\lib\SILGen\SILGenProfiling.cpp)

The fix is to include Expr.h in Stmt.h.

Alternatively, we include Expr.h in every single file where Stmt.h is included, but this is a longer change and less scalable.

[hughbe]

@swift-ci please smoke test

[slavapestov]

Does this increase the number of files that transitively include Expr.h? Last I checked it was around 170. Of its not much worse then this LGTM :)

[hughbe]

I did the following:

• Fully compile swift (ninja swift)
• Edit Expr.h
• Recompile swift (ninja swift)

171 files needed recompilation.

[hughbe]

FYI: https://developercommunity.visualstudio.com/content/problem/30918/regression-compiler-intrinsic-type-trait-with-forw.html

This is likely MSVC actually doing the right thing now, but is still a source breaking change that should be documented, or allowed when the /permissive- flag is not specified