that is in an anonymous namespace, give that function or variable
internal linkage.

This change models an oddity of the C++ standard, where names declared
in an anonymous namespace have external linkage but, because anonymous
namespace are really "uniquely-named" namespaces, the names cannot be
referenced from other translation units. That means that they have
external linkage for semantic analysis, but the only sensible
implementation for code generation is to give them internal
linkage. We now model this notion via the UniqueExternalLinkage
linkage type. There are several changes here:

  - Extended NamedDecl::getLinkage() to produce UniqueExternalLinkage
    when the declaration is in an anonymous namespace.
  - Added Type::getLinkage() to determine the linkage of a type, which
    is defined as the minimum linkage of the types (when we're dealing
    with a compound type that is not a struct/class/union).
  - Extended NamedDecl::getLinkage() to consider the linkage of the
    template arguments and template parameters of function template
    specializations and class template specializations.
  - Taught code generation to rely on NamedDecl::getLinkage() when
    determining the linkage of variables and functions, also
    considering the linkage of the types of those variables and
    functions (C++ only). Map UniqueExternalLinkage to internal
    linkage, taking out the explicit checks for
    isInAnonymousNamespace().

This fixes much of PR5792, which, as discovered by Anders Carlsson, is
actually the reason behind the pass-manager assertion that causes the
majority of clang-on-clang regression test failures. With this fix,
Clang-built-Clang+LLVM passes 88% of its regression tests (up from
67%). The specific numbers are:

LLVM:
  Expected Passes    : 4006
  Expected Failures  : 32
  Unsupported Tests  : 40
  Unexpected Failures: 736

Clang:
  Expected Passes    : 1903
  Expected Failures  : 14
  Unexpected Failures: 75

Overall:
  Expected Passes    : 5909
  Expected Failures  : 46
  Unsupported Tests  : 40
  Unexpected Failures: 811

Still to do:
  - Improve testing
  - Check whether we should allow the presence of types with
  InternalLinkage (in addition to UniqueExternalLinkage) given
  variables/functions internal linkage in C++, as mentioned in
  PR5792. 
  - Determine how expensive the getLinkage() calls are in practice;
  consider caching the result in NamedDecl.
  - Assess the feasibility of Chris's idea in comment #1 of PR5792.

llvm-svn: 95216