[clang-tools-extra] 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: 368944
diff --git a/clang-tools-extra/clangd/Headers.cpp b/clang-tools-extra/clangd/Headers.cpp
index 7caaeba..525f770 100644
--- a/clang-tools-extra/clangd/Headers.cpp
+++ b/clang-tools-extra/clangd/Headers.cpp
@@ -112,7 +112,7 @@
 std::unique_ptr<PPCallbacks>
 collectIncludeStructureCallback(const SourceManager &SM,
                                 IncludeStructure *Out) {
-  return llvm::make_unique<RecordHeaders>(SM, Out);
+  return std::make_unique<RecordHeaders>(SM, Out);
 }
 
 void IncludeStructure::recordInclude(llvm::StringRef IncludingName,