[Sema] Allow unmarked overloadable functions.


[Sema] Allow unmarked overloadable functions.

This patch extends the overloadable attribute to allow for one
function with a given name to not be marked with the overloadable
attribute. The overload without the overloadable attribute will not
have its name mangled.

So, the following code is now legal:

void foo(void) __attribute__((overloadable));
void foo(int);
void foo(float) __attribute__((overloadable));

In addition, this patch fixes a bug where we'd accept code with
__attribute__((overloadable)) inconsistently applied. In other words,
we used to accept:

void foo(void);
void foo(void) __attribute__((overloadable));

But we will do this no longer, since it defeats the original purpose of
requiring __attribute__((overloadable)) on all redeclarations of a

This breakage seems to not be an issue in practice, since the only code
I could find that had this pattern often looked like:

void foo(void);
void foo(void) __attribute__((overloadable)) __asm__("foo");
void foo(int) __attribute__((overloadable));

...Which can now be simplified by simply removing the asm label and
overloadable attribute from the redeclaration of void foo(void);

Differential Revision: https://reviews.llvm.org/D32332