Introduce _Complex conversions into the function overloading

system. Since C99 doesn't have overloading and C++ doesn't have
_Complex, there is no specification for    this. Here's what I think
makes sense.

Complex conversions come in several flavors:

  - Complex promotions:  a complex -> complex   conversion where the
    underlying real-type conversion is a floating-point promotion. GCC
    seems to call this a promotion, EDG does something else. This is
    given "promotion" rank for determining the best viable function.
  - Complex conversions: a complex -> complex conversion that is
    not a complex promotion. This is given "conversion" rank for
    determining the best viable   function.
  - Complex-real conversions: a real -> complex or complex -> real
    conversion. This is given "conversion" rank for determining the
    best viable function.

These rules are the same for C99 (when using the "overloadable"
attribute) and C++. However, there is one difference in the handling
of floating-point promotions: in C99, float -> long double and double
-> long double are considered promotions (so we give them "promotion" 
rank), while C++ considers these conversions ("conversion" rank).

llvm-svn: 64343