[llvm] [z/OS] Add z/OS archive reading support (PR #187110)

[Uyiosa Iyekekpolor via llvm-commits]

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@@ -434,6 +472,18 @@ class BigArchive : public Archive {
   bool has64BitGlobalSymtab() { return Has64BitGlobalSymtab; }
 };
 
+class ZOSArchive : public Archive {
+public:
+  // Fixed-Length header.
+  struct FixLenHdr {
+    char Magic[sizeof(ZOSArchiveMagic) - 1]; ///< ZOS archive magic string.
+  };
+
+  ZOSArchive(MemoryBufferRef Source, Error &Err);
+
+private:
+  std::string SymbolTableBuf; // __.SYMDEF strings converted to ASCII.
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uyoyo0 wrote:

Its pretty much just used for memory management.

In the constructor, after we convert the EBCDIC names to ASCII, we append them to SymbolTableBuf which is a std::string. We then set the base class's SymbolTable to point to it: 
```
SymbolTable = StringRef(SymbolTableBuf.data(), SymbolTableBuf.size());
```

Because `Archive::SymbolTable` is just a `StringRef`, it doesn't own the memory. Therefore, `SymbolTableBuf` needs to be a class member so that the backing buffer stays alive for the entire lifetime of the `ZOSArchive` object.

https://github.com/llvm/llvm-project/pull/187110