Revert "[LoongArch] Change 'using namespace llvm;' to 'namespace llvm {' in LoongArchTargetParser.cpp. NFC"

This reverts commit 5863214.

Reason to revert:
These changes are inconsistent with the [[https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-implement-previously-declared-functions|LLVM stype guide]].

Author: SixWeining

llvm/lib/TargetParser/LoongArchTargetParser.cpp

@@ -13,8 +13,8 @@
 
 #include "llvm/TargetParser/LoongArchTargetParser.h"
 
-namespace llvm {
-namespace LoongArch {
+using namespace llvm;
+using namespace llvm::LoongArch;
 
 const FeatureInfo AllFeatures[] = {
 #define LOONGARCH_FEATURE(NAME, KIND) {NAME, KIND},
@@ -27,14 +27,15 @@ const ArchInfo AllArchs[] = {
 #include "llvm/TargetParser/LoongArchTargetParser.def"
 };
 
-bool isValidArchName(StringRef Arch) {
+bool LoongArch::isValidArchName(StringRef Arch) {
   for (const auto A : AllArchs)
     if (A.Name == Arch)
       return true;
   return false;
 }
 
-bool getArchFeatures(StringRef Arch, std::vector<StringRef> &Features) {
+bool LoongArch::getArchFeatures(StringRef Arch,
+                                std::vector<StringRef> &Features) {
   for (const auto A : AllArchs) {
     if (A.Name == Arch) {
       for (const auto F : AllFeatures)
@@ -45,5 +46,3 @@ bool getArchFeatures(StringRef Arch, std::vector<StringRef> &Features) {
   }
   return false;
 }
-} // namespace LoongArch
-} // namespace llvm