[OffloadBundler] Rework the ctor of OffloadTargetInfo to support AMDGPU's generic target (#122629)

The current parsing logic for the target string assumes it follows the
format `<kind>-<triple>-<target id>:<feature>`, such as
`hipv4-amdgcn-amd-amdhsa-gfx1030:+xnack`.
Specifically, it assumes that `<target id>` does not contain any `-`,
relying on `rsplit` for parsing.
However, this assumption breaks for AMDGPU's generic targets, which may
contain one or more `-`, such as `gfx10-3-generic` or `gfx12-generic`.
As a result, the existing approach using `rstrip` is no longer reliable.

This patch reworks the parsing logic to handle target strings more
robustly, including support for generic targets.
The bundler now strictly requires a 4-field target triple.
Additionally, a new Python helper function has been added to `config.py`
to normalize the target triple into the 4-field format when it is not,
ensuring tests pass reliably.

Author: shiltian

lit/llvm/config.py

@@ -355,6 +355,14 @@ def get_clang_has_lsan(self, clang, triple):
 
         return False
 
+    # Normalize 3-field target triple to 4-field triple with "unknown" as environment
+    def normalize_triple(self, triple):
+        compoments = triple.split("-", maxsplit=3)
+        if len(compoments) == 4:
+            return triple
+        assert len(compoments) == 3
+        return triple + "-unknown"
+
     def make_itanium_abi_triple(self, triple):
         m = re.match(r"(\w+)-(\w+)-(\w+)", triple)
         if not m:
@@ -665,7 +673,9 @@ def use_clang(
             self.config.substitutions.append(
                 (
                     "%itanium_abi_triple",
-                    self.make_itanium_abi_triple(self.config.target_triple),
+                    self.normalize_triple(
+                        self.make_itanium_abi_triple(self.config.target_triple)
+                    ),
                 )
             )
             self.config.substitutions.append(