[DebugInfo] Add calling conventions to DISubroutineType

Summary:
This should have been a very simple change, but it was greatly
complicated by the construction of new Decls during IR generation.

In particular, we reconstruct the AST function type in order to get the
implicit 'this' parameter into C++ method types.

We also have to worry about FunctionDecls whose types are not
FunctionTypes because CGBlocks.cpp constructs some dummy FunctionDecls
with 'void' type.

Depends on D21114

Reviewers: aprantl, dblaikie

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21141

llvm-svn: 272198