[CodeGen] Unify generation of scalar and struct-path TBAA tags

This patch makes it possible to produce access tags in a uniform
manner regardless whether the resulting tag will be a scalar or a
struct-path one. getAccessTagInfo() now takes care of the actual
translation of access descriptors to tags and can handle all
kinds of accesses. Facilities that specific to scalar accesses
are eliminated.

Some more details:
* DecorateInstructionWithTBAA() is not responsible for conversion
  of types to access tags anymore. Instead, it takes an access
  descriptor (TBAAAccessInfo) and generates corresponding access
  tag from it.
* getTBAAInfoForVTablePtr() reworked to
  getTBAAVTablePtrAccessInfo() that now returns the
  virtual-pointer access descriptor and not the virtual-point
  type metadata.
* Added function getTBAAMayAliasAccessInfo() that returns the
  descriptor for may-alias accesses.
* getTBAAStructTagInfo() renamed to getTBAAAccessTagInfo() as now
  it is the only way to generate access tag by a given access
  descriptor. It is capable of producing both scalar and
  struct-path tags, depending on options and availability of the
  base access type. getTBAAScalarTagInfo() and its cache
  ScalarTagMetadataCache are eliminated.
* Now that we do not need to care about whether the resulting
  access tag should be a scalar or struct-path one,
  getTBAAStructTypeInfo() is renamed to getBaseTypeInfo().
* Added function getTBAAAccessInfo() that constructs access
  descriptor by a given QualType access type.

This is part of D37826 reworked to be a separate patch to
simplify review.

Differential Revision: https://reviews.llvm.org/D38503

llvm-svn: 314977