[clang] [Clang][Driver] Ensure clang can construct toolchain target names (PR #115798)

[Matthew Mirvish via cfe-commits]

https://github.com/mincrmatt12 created https://github.com/llvm/llvm-project/pull/115798

This adds an extra validation step to `ToolChain::getTargetAndModeFromProgramName` to actually try constructing the target info for the triple it deduces, instead of just relying on the LLVM target registry.

Some targets, like `xtensa`, are supported by LLVM but are not supported by clang, so just checking the LLVM registry causes inconsistency.

This is based on the discussion from clangd/clangd#2184 (and indeed would solve that issue) - the approach here of "construct the target info and check for null" is taken from [clangd](https://github.com/llvm/llvm-project/blob/5716f836d25e93bf8f664a14fe55c70e07a369be/clang-tools-extra/clangd/SystemIncludeExtractor.cpp#L253).

>From f111040a50db8275b1d6aaee3cf5e4b8af2b08f5 Mon Sep 17 00:00:00 2001
From: Matthew Mirvish <matthew at mm12.xyz>
Date: Mon, 11 Nov 2024 20:20:20 -0500
Subject: [PATCH] [Clang][Driver] Ensure clang can construct toolchain target
 names

This adds an extra validation step to
`ToolChain::getTargetAndModeFromProgramName` to actually try
constructing the target info for the triple it deduces, instead of just
relying on the LLVM target registry.

Some targets, like `xtensa`, are supported by LLVM but are not supported
by clang, so just checking the LLVM registry causes inconsistency.
---
 clang/lib/Driver/ToolChain.cpp | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 646dbc0faf5a9f..e094a5fd0267f3 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -16,6 +16,8 @@
 #include "ToolChains/InterfaceStubs.h"
 #include "clang/Basic/ObjCRuntime.h"
 #include "clang/Basic/Sanitizers.h"
+#include "clang/Basic/TargetInfo.h"
+#include "clang/Basic/TargetOptions.h"
 #include "clang/Config/config.h"
 #include "clang/Driver/Action.h"
 #include "clang/Driver/Driver.h"
@@ -496,8 +498,21 @@ ToolChain::getTargetAndModeFromProgramName(StringRef PN) {
   StringRef Prefix(ProgName);
   Prefix = Prefix.slice(0, LastComponent);
   std::string IgnoredError;
+
+  // First, check if the target is a supported LLVM target.
   bool IsRegistered =
       llvm::TargetRegistry::lookupTarget(std::string(Prefix), IgnoredError);
+  // If so, further check that clang is able to use it as a target.
+  if (IsRegistered) {
+    std::shared_ptr<TargetOptions> TargetOpts(new TargetOptions);
+    TargetOpts->Triple = Prefix.str();
+    DiagnosticsEngine Diags(new DiagnosticIDs, new DiagnosticOptions,
+                            new IgnoringDiagConsumer);
+    llvm::IntrusiveRefCntPtr<TargetInfo> Target =
+        clang::TargetInfo::CreateTargetInfo(Diags, TargetOpts);
+    if (!Target)
+      IsRegistered = false;
+  }
   return ParsedClangName{std::string(Prefix), ModeSuffix, DS->ModeFlag,
                          IsRegistered};
 }