[clang] Change std::sort to llvm::sort in response to r327219

r327219 added wrappers to std::sort which randomly shuffle the container before
sorting.  This will help in uncovering non-determinism caused due to undefined
sorting order of objects having the same key.

To make use of that infrastructure we need to invoke llvm::sort instead of
std::sort.

llvm-svn: 328636
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 1c3495a..77c06f2 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -2186,7 +2186,7 @@
       }
     }
 
-    std::sort(
+    llvm::sort(
         Bases.begin(), Bases.end(), [&](const std::pair<QualType, int64_t> &L,
                                         const std::pair<QualType, int64_t> &R) {
           return Layout.getBaseClassOffset(L.first->getAsCXXRecordDecl()) <