Fix typo in assertion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179061 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaLookup.cpp

@@ -3405,7 +3405,7 @@ class NamespaceSpecifierSet {
 }
 
 DeclContextList NamespaceSpecifierSet::BuildContextChain(DeclContext *Start) {
-  assert(Start && "Bulding a context chain from a null context");
+  assert(Start && "Building a context chain from a null context");
   DeclContextList Chain;
   for (DeclContext *DC = Start->getPrimaryContext(); DC != NULL;
        DC = DC->getLookupParent()) {