[clang] [clang][driver] Fix flang frontend resolution for symlinked clang driver (PR #204253)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-driver

Author: Haowei (zeroomega)

<details>
<summary>Changes</summary>

When clang is built as a symlink to the llvm multicall binary, the driver resolves the executable path to 'llvm', making D.Name 'llvm' instead of 'clang'.
This broke the detection of 'clang --driver-mode=flang' in Flang.cpp, causing the driver to try to run 'llvm -fc1' instead of looking up 'flang', after change in #<!-- -->200438 was landed.

Fix this by checking D.getPrependArg() in addition to D.Name to identify if we are acting as clang. Also use D.IsFlangMode() for simpler and more robust detection of the driver mode.

---
Full diff: https://github.com/llvm/llvm-project/pull/204253.diff


1 Files Affected:

- (modified) clang/lib/Driver/ToolChains/Flang.cpp (+5-4) 


``````````diff
diff --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
index 3e8adbb39349b..aed572c32ccd6 100644
--- a/clang/lib/Driver/ToolChains/Flang.cpp
+++ b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -1328,10 +1328,11 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
 
   // Handle "clang --driver-mode=flang" case
   bool isClangDriverWithFlangMode = false;
-  if (D.Name.find("clang") != std::string_view::npos)
-    if (const Arg *A = Args.getLastArg(options::OPT_driver_mode))
-      if (StringRef(A->getValue()) == "flang")
-        isClangDriverWithFlangMode = true;
+  std::string DriverName = D.Name;
+  if (const char *PA = D.getPrependArg())
+    DriverName = PA;
+  if (DriverName.find("clang") != std::string::npos && D.IsFlangMode())
+    isClangDriverWithFlangMode = true;
 
   const char *Exec = isClangDriverWithFlangMode
                          ? Args.MakeArgString(D.GetProgramPath("flang", TC))

``````````

</details>


https://github.com/llvm/llvm-project/pull/204253