[AST][NFC] Pass the AST context to one of the ctor of DeclRefExpr.

All of the other constructors already take a reference to the AST context.
This avoids calling Decl::getASTContext in most cases. Additionally move
the definition of the constructor from Expr.h to Expr.cpp since it is calling
DeclRefExpr::computeDependence. NFC.

llvm-svn: 349901