[AST][3/4] Move the bit-fields from BlockDecl, LinkageSpecDecl and...

[AST][3/4] Move the bit-fields from BlockDecl, LinkageSpecDecl and OMPDeclareReductionDecl into DeclContext

This patch follows https://reviews.llvm.org/D49729
and https://reviews.llvm.org/D49732, and is
followed by https://reviews.llvm.org/D49734.

Move the bits from BlockDecl, LinkageSpecDecl and
OMPDeclareReductionDecl into DeclContext.

Differential Revision: https://reviews.llvm.org/D49733

Patch By: bricci

llvm-svn: 338639