[clang] [clang AST] move mangling API to namespace clang to allow calls from swift-frontend (PR #137884)

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<!--LLVM CODE FORMAT COMMENT: {clang-format}-->


:warning: C/C++ code formatter, clang-format found issues in your code. :warning:

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You can test this locally with the following command:
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``````````bash
git-clang-format --diff HEAD~1 HEAD --extensions cpp,h -- clang/include/clang/AST/Mangle.h clang/lib/AST/Mangle.cpp
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View the diff from clang-format here.
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``````````diff
diff --git a/clang/include/clang/AST/Mangle.h b/clang/include/clang/AST/Mangle.h
index 1afbf80df..ca72dcfd4 100644
--- a/clang/include/clang/AST/Mangle.h
+++ b/clang/include/clang/AST/Mangle.h
@@ -43,9 +43,9 @@ class VarDecl;
 /// Extract mangling function name from MangleContext such that swift can call
 /// it to prepare for ObjCDirect in swift.
 void mangleObjCMethodName(raw_ostream &OS, bool includePrefixByte,
-  bool isInstanceMethod, StringRef ClassName,
-  std::optional<StringRef> CategoryName,
-  StringRef MethodName);
+                          bool isInstanceMethod, StringRef ClassName,
+                          std::optional<StringRef> CategoryName,
+                          StringRef MethodName);
 
 /// MangleContext - Context for tracking state which persists across multiple
 /// calls to the C++ name mangler.

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https://github.com/llvm/llvm-project/pull/137884