Fix version variable check. (#576)

Exactly one must be present.

This addresses:
#575

Author: helly25

toolchain/rules.bzl

@@ -38,8 +38,12 @@ toolchain = repository_rule(
 )
 
 def llvm_toolchain(name, **kwargs):
-    if kwargs.get("llvm_version") == kwargs.get("llvm_versions"):
+    if kwargs.get("llvm_version") and kwargs.get("llvm_versions"):
         fail("Exactly one of llvm_version or llvm_versions must be set")
+    if not kwargs.get("llvm_versions"):
+        if not kwargs.get("llvm_version"):
+            fail("One of llvm_version or llvm_versions must be set")
+        kwargs.update(llvm_versions = {"": kwargs.get("llvm_version")})
 
     if not kwargs.get("toolchain_roots"):
         llvm_args = {
@@ -49,9 +53,6 @@ def llvm_toolchain(name, **kwargs):
         }
         llvm(name = name + "_llvm", **llvm_args)
 
-    if not kwargs.get("llvm_versions"):
-        kwargs.update(llvm_versions = {"": kwargs.get("llvm_version")})
-
     toolchain_args = {
         k: v
         for k, v in kwargs.items()