Initial implementation of function overloading in C.

This commit adds a new attribute, "overloadable", that enables C++
function overloading in C. The attribute can only be added to function
declarations, e.g.,

  int *f(int) __attribute__((overloadable));

If the "overloadable" attribute exists on a function with a given
name, *all* functions with that name (and in that scope) must have the
"overloadable" attribute. Sets of overloaded functions with the
"overloadable" attribute then follow the normal C++ rules for
overloaded functions, e.g., overloads must have different
parameter-type-lists from each other.

When calling an overloaded function in C, we follow the same
overloading rules as C++, with three extensions to the set of standard
conversions:

  - A value of a given struct or union type T can be converted to the
    type T. This is just the identity conversion. (In C++, this would
    go through a copy constructor).
  - A value of pointer type T* can be converted to a value of type U*
    if T and U are compatible types. This conversion has Conversion
    rank (it's considered a pointer conversion in C).
  - A value of type T can be converted to a value of type U if T and U
    are compatible (and are not both pointer types). This conversion
    has Conversion rank (it's considered to be a new kind of
    conversion unique to C, a "compatible" conversion).

Known defects (and, therefore, next steps):
  1) The standard-conversion handling does not understand conversions
  involving _Complex or vector extensions, so it is likely to get
  these wrong. We need to add these conversions.
  2) All overloadable functions with the same name will have the same
  linkage name, which means we'll get a collision in the linker (if
  not sooner). We'll need to mangle the names of these functions.

llvm-svn: 64336