[OpenCL] Print builtin function prototypes if ambiguous

[Pierre]

Summary

When hitting an ambigous call to a builtin function with the
-fdeclare-opencl-builtins option, diagnostics don't print the prototypes
that clash.
When not using the option above, they are displayed.
This patch prints them.

This is changing this diagnostic:

test.cl:86:11: error: call to 'acos' is ambiguous
  int a = acos(p);
          ^~~~
test.cl:86:11: note: candidate function
test.cl:86:11: note: candidate function
[not printing everything ...]
test.cl:86:11: note: candidate function
1 error generated.

To this:

test.cl:86:11: error: call to 'acos' is ambiguous
  int a = acos(p);
          ^~~~
test.cl:86:11: note: candidate function
float acos(float)
test.cl:86:11: note: candidate function
double acos(double)
[not printing everything ...]
test.cl:86:11: note: candidate function
__fp16 __attribute__((ext_vector_type(16))) acos(__fp16 __attribute__((ext_vector_type(16))))
1 error generated.

clang/lib/Sema/SemaOverload.cpp

 
     // We don't really have anything else to say about viable candidates.
     S.NoteOverloadCandidate(Cand->FoundDecl, Fn);
+
+    // If this is a builtin function, give the available definitions.
+    if (S.getLangOpts().OpenCL && Fn->isImplicit()) {
+      raw_ostream &OS = llvm::outs();
+      unsigned NumParams = Fn->getNumParams();
+
+      OS << Fn->getReturnType().getAsString() << " ";
+      OS << Fn->getNameInfo().getAsString() << "(";
+
+      if (NumParams > 0) {
+        OS << Fn->getParamDecl(0)->getOriginalType().getAsString();
+      }
+      for (unsigned i = 1; i < NumParams; i++) {
+        OS << ", ";
+        OS << Fn->getParamDecl(i)->getOriginalType().getAsString();
+      }
+      OS << ")\n";
+    }
+
     return;
   }