[lld][WebAssembly] Use C++17 nested namespace syntax in most places. NFC

Like D131405, but for wasm-ld.

Differential Revision: https://reviews.llvm.org/D145399
diff --git a/lld/wasm/Relocations.cpp b/lld/wasm/Relocations.cpp
index c7710a9..2f6dd6a 100644
--- a/lld/wasm/Relocations.cpp
+++ b/lld/wasm/Relocations.cpp
@@ -16,8 +16,7 @@
 using namespace llvm;
 using namespace llvm::wasm;
 
-namespace lld {
-namespace wasm {
+namespace lld::wasm {
 
 static bool requiresGOTAccess(const Symbol *sym) {
   if (!config->isPic &&
@@ -176,5 +175,4 @@
   }
 }
 
-} // namespace wasm
-} // namespace lld
+} // namespace lld::wasm