[llvm] Add StringMap::lookup() overload to take a default value and return a reference to the value to avoid a copy (PR #115469)

[Fangrui Song via llvm-commits]

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@@ -257,6 +257,15 @@ class LLVM_ALLOCATORHOLDER_EMPTYBASE StringMap
     return ValueTy();
   }
 
+  /// lookup - Return the entry for the specified key, or a default
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MaskRay wrote:

Remove `lookup -`　when adding new methods

https://llvm.org/docs/CodingStandards.html#doxygen-use-in-documentation-comments
Don’t duplicate the documentation comment in the header file and in the implementation file. 

https://github.com/llvm/llvm-project/pull/115469