[llvm] [LLVM][Triple] Drop unknown object types from normalized triples (PR #135571)

[Usman Nadeem via llvm-commits]

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@@ -1265,6 +1266,14 @@ std::string Triple::normalize(StringRef Str, CanonicalForm Form) {
     }
   }
 
+  // Environment "unknown-elf" is just "elf".
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UsmanNadeem wrote:

`parseEnvironment("elf")` will also yield `UnknownEnvironment` though.

Before this patch the following command 
`clang -### --target=aarch64-pc-linux--elf` or `clang -### --target=aarch64-pc-linux--elf` gives an error due to bug in version parsing but the resultant triple is:
`Target: aarch64-pc-linux-elf-unknown` which seems wrong.

After this patch we get:
`Target: aarch64-pc-linux-elf`


https://github.com/llvm/llvm-project/pull/135571