[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/Basic/VirtualFileSystem.cpp b/clang/lib/Basic/VirtualFileSystem.cpp
index 9d44597..e95b32a 100644
--- a/clang/lib/Basic/VirtualFileSystem.cpp
+++ b/clang/lib/Basic/VirtualFileSystem.cpp
@@ -1812,8 +1812,8 @@
 }
 
 void YAMLVFSWriter::write(llvm::raw_ostream &OS) {
-  std::sort(Mappings.begin(), Mappings.end(),
-            [](const YAMLVFSEntry &LHS, const YAMLVFSEntry &RHS) {
+  llvm::sort(Mappings.begin(), Mappings.end(),
+             [](const YAMLVFSEntry &LHS, const YAMLVFSEntry &RHS) {
     return LHS.VPath < RHS.VPath;
   });