[AST][1/4] Move the bit-fields from TagDecl, EnumDecl and RecordDecl into DeclContext

DeclContext has a little less than 8 bytes free due to the alignment
requirements on 64 bits archs. This set of patches moves the
bit-fields from classes deriving from DeclContext into DeclContext.

On 32 bits archs this increases the size of DeclContext by 4 bytes
but this is balanced by an equal or larger reduction in the size
of the classes deriving from it.

On 64 bits archs the size of DeclContext stays the same but
most of the classes deriving from it shrink by 8/16 bytes.
(-print-stats diff here https://reviews.llvm.org/D49728)
When doing an -fsyntax-only on all of Boost this result
in a 3.6% reduction in the size of all Decls and
a 1% reduction in the run time due to the lower cache
miss rate.

For now CXXRecordDecl is not touched but there is
an easy 6 (if I count correctly) bytes gain available there
by moving some bits from DefinitionData into the free
space of DeclContext. This will be the subject of another patch.

This patch sequence also enable the possibility of refactoring
FunctionDecl: To save space some bits from classes deriving from
FunctionDecl were moved to FunctionDecl. This resulted in a
lot of stuff in FunctionDecl which do not belong logically to it.
After this set of patches however it is just a simple matter of
adding a SomethingDeclBitfields in DeclContext and moving the
bits to it from FunctionDecl.

This first patch introduces the anonymous union in DeclContext
and all the *DeclBitfields classes holding the bit-fields, and moves
the bits from TagDecl, EnumDecl and RecordDecl into DeclContext.

This patch is followed by https://reviews.llvm.org/D49732,
https://reviews.llvm.org/D49733 and https://reviews.llvm.org/D49734.

Differential Revision: https://reviews.llvm.org/D49729

Patch By: bricci

llvm-svn: 338630