Improve merging of function declarations. Specifically:

  - When we are declaring a function in local scope, we can merge with
    a visible declaration from an outer scope if that declaration
    refers to an entity with linkage. This behavior now works in C++
    and properly ignores entities without linkage.
  - Diagnose the use of "static" on a function declaration in local
    scope.
  - Diagnose the declaration of a static function after a non-static
    declaration of the same function.
  - Propagate the storage specifier to a function declaration from a
    prior declaration (PR3425)
  - Don't name-mangle "main"

llvm-svn: 65360