[Parse] Improve diagnostic for no platform condition argument

Drop the "only one" in the message in this case,
as that should only apply for too many args.

Author: hamishknight

include/swift/AST/DiagnosticsParse.def

@@ -1751,6 +1751,9 @@ ERROR(unsupported_platform_condition_expression,none,
       "unexpected platform condition "
       "(expected 'os', 'arch', or 'swift')",
       ())
+ERROR(platform_condition_expected_argument,none,
+      "expected argument to platform condition",
+      ())
 ERROR(platform_condition_expected_one_argument,none,
       "expected only one argument to platform condition",
       ())

lib/Parse/ParseIfConfig.cpp

@@ -263,7 +263,11 @@ class ValidateIfConfigCondition :
 
     Expr *Arg = getSingleSubExp(E->getArgs(), *KindName, &D);
     if (!Arg) {
-      D.diagnose(E->getLoc(), diag::platform_condition_expected_one_argument);
+      if (E->getArgs()->empty()) {
+        D.diagnose(E->getLoc(), diag::platform_condition_expected_argument);
+      } else {
+        D.diagnose(E->getLoc(), diag::platform_condition_expected_one_argument);
+      }
       return nullptr;
     }
     // '_compiler_version' '(' string-literal ')'

test/Parse/ConditionalCompilation/basicParseErrors.swift

@@ -98,7 +98,7 @@ func fn_j() {}
 #endif
 fn_j() // OK
 
-#if foo || bar || nonExistent() // expected-error {{expected only one argument to platform condition}}
+#if foo || bar || nonExistent() // expected-error {{expected argument to platform condition}}
 #endif
 
 #if FOO = false
@@ -171,5 +171,5 @@ if true {}
 
 // rdar://83017601 Make sure we don't crash
 #if canImport()
-// expected-error@-1 {{expected only one argument to platform condition}}
+// expected-error@-1 {{expected argument to platform condition}}
 #endif