[Clang] Migrate llvm::make_unique to std::make_unique

Now that we've moved to C++14, we no longer need the llvm::make_unique
implementation from STLExtras.h. This patch is a mechanical replacement
of (hopefully) all the llvm::make_unique instances across the monorepo.

Differential revision: https://reviews.llvm.org/D66259

llvm-svn: 368942
diff --git a/clang/unittests/AST/ExternalASTSourceTest.cpp b/clang/unittests/AST/ExternalASTSourceTest.cpp
index 3a0fe01..ba8a8cd 100644
--- a/clang/unittests/AST/ExternalASTSourceTest.cpp
+++ b/clang/unittests/AST/ExternalASTSourceTest.cpp
@@ -37,7 +37,7 @@
 
   std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI,
                                                  StringRef InFile) override {
-    return llvm::make_unique<ASTConsumer>();
+    return std::make_unique<ASTConsumer>();
   }
 
   IntrusiveRefCntPtr<ExternalASTSource> Source;