[TableGen] Ignore fake args for parsing-related arg counts.

We should complain about the following:

```
void foo() __attribute__((unavailable("a", "b")));
```

Instead, we currently just ignore "b". (...We also end up ignoring "a",
because we assume elsewhere that this attribute can only have 1 or 0
args.)

This happens because `unavailable` has a fake enum arg, and
`AttributeList::{getMinArgs,getMaxArgs}` include fake args in their
counts.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288388 91177308-0d34-0410-b5e6-96231b3b80d8

test/Sema/attr-unavailable-message.c

@@ -1,6 +1,8 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
 // rdar: //6734520
 
+void tooManyArgs() __attribute__((unavailable("a", "b"))); // expected-error {{'unavailable' attribute takes no more than 1 argument}}
+
 int foo(int)  __attribute__((__unavailable__("USE IFOO INSTEAD"))); // expected-note {{'foo' has been explicitly marked unavailable here}}
 double dfoo(double)  __attribute__((__unavailable__("NO LONGER"))); // expected-note 2 {{'dfoo' has been explicitly marked unavailable here}}
 

utils/TableGen/ClangAttrEmitter.cpp

@@ -2540,6 +2540,10 @@ static void emitArgInfo(const Record &R, std::stringstream &OS) {
   unsigned ArgCount = 0, OptCount = 0;
   bool HasVariadic = false;
   for (const auto *Arg : Args) {
+    // If the arg is fake, it's the user's job to supply it: general parsing
+    // logic shouldn't need to know anything about it.
+    if (Arg->getValueAsBit("Fake"))
+      continue;
     Arg->getValueAsBit("Optional") ? ++OptCount : ++ArgCount;
     if (!HasVariadic && isArgVariadic(*Arg, R.getName()))
       HasVariadic = true;