Re-implement caching for the linkage calculation of declarations.

My previous attempt at solving the compile-time problem with many
redeclarations of the same entity cached both linkage and visibility,
while this patch only tackles linkage. There are several reasons for
this difference:

  - Linkage is a language concept, and is evaluated many times during
    semantic analysis and codegen, while visibility is only a
    code-generation concept that is evaluated only once per (unique)
    declaration. Hence, we *must* optimize linkage calculations but
    don't need to optimize visibility computation.
  - Once we know the linkage of a declaration, subsequent
    redeclarations can't change that linkage. Hence, cache
    invalidation is far simpler than for visibility, where a later
    redeclaration can completely change the visibility.
  - We have 3 spare bits in Decl to store the linkage cache, so the
    cache doesn't increase the size of declarations. With the
    visibility+linkage cache, NamedDecl got larger.

llvm-svn: 121023