[ELF] Wrap things in `namespace lld { namespace elf {`, NFC

This makes it clear `ELF/**/*.cpp` files define things in the `lld::elf`
namespace and simplifies `elf::foo` to `foo`.

Reviewed By: atanasyan, grimar, ruiu

Differential Revision: https://reviews.llvm.org/D68323

llvm-svn: 373885
diff --git a/lld/ELF/CallGraphSort.cpp b/lld/ELF/CallGraphSort.cpp
index 459aa3c..6f8ef89 100644
--- a/lld/ELF/CallGraphSort.cpp
+++ b/lld/ELF/CallGraphSort.cpp
@@ -48,8 +48,9 @@
 #include <numeric>
 
 using namespace llvm;
-using namespace lld;
-using namespace lld::elf;
+
+namespace lld {
+namespace elf {
 
 namespace {
 struct Edge {
@@ -264,6 +265,9 @@
 // This first builds a call graph based on the profile data then merges sections
 // according to the C³ huristic. All clusters are then sorted by a density
 // metric to further improve locality.
-DenseMap<const InputSectionBase *, int> elf::computeCallGraphProfileOrder() {
+DenseMap<const InputSectionBase *, int> computeCallGraphProfileOrder() {
   return CallGraphSort().run();
 }
+
+} // namespace elf
+} // namespace lld