Encapsulate within CXCursor the notion of whether a VarDecl* is the first Decl in...

Encapsulate within CXCursor the notion of whether a VarDecl* is the first Decl in a DeclGroup.  We use this
to recover some context that is currently not modeled directly in the AST.  Currently VarDecl's cannot
properly determine their source range because they have no context on whether or not they appear in a DeclGroup.
For the meantime, this bandaid suffices in libclang since that is where the correct SourceRange is directly needed.

Fixes <rdar://problem/8595749>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117973 91177308-0d34-0410-b5e6-96231b3b80d8