Rework Microsoft __if_exists/__if_not_exists parsing and semantic

analysis to separate dependent names from non-dependent names. For
dependent names, we'll behave differently from Visual C++:

  - For __if_exists/__if_not_exists at class scope, we'll just warn
    and then ignore them.
  - For __if_exists/__if_not_exists in statements, we'll treat the
    inner statement as a compound statement, which we only instantiate
    in templates where the dependent name (after instantiation)
    exists. This behavior is different from VC++, but it's as close as
    we can get without encroaching ridiculousness.

The latter part (dependent statements) is not yet implemented.

llvm-svn: 142864