[clang] [llvm] Define LLVM_ABI and CLANG_ABI for __EMSCRIPTEN__ builds (PR #131578)

[Anutosh Bhat via llvm-commits]

https://github.com/anutosh491 created https://github.com/llvm/llvm-project/pull/131578

While building llvm (clang, lld) against emscripten we see this [error](https://github.com/emscripten-forge/recipes/actions/runs/13803029307/job/38608794602#step:9:1715)

```
 │ │ In file included from $SRC_DIR/llvm/lib/Frontend/OpenACC/ACC.cpp:9:
 │ │ $SRC_DIR/build/include/llvm/Frontend/OpenACC/ACC.h.inc:192:1: error: unknown type name 'LLVM_ABI'
 │ │   192 | LLVM_ABI Directive getOpenACCDirectiveKind(llvm::StringRef Str);
 │ │       | ^
 │ │ $SRC_DIR/build/include/llvm/Frontend/OpenACC/ACC.h.inc:192:19: error: expected ';' after top level declarator
 │ │   192 | LLVM_ABI Directive getOpenACCDirectiveKind(llvm::StringRef Str);
 │ │       |                   ^
 ```

Now this was happening because we weren't defining LLVM_ABI correctly when building against emscripten. If you see [llvm/Support/Compiler.h](https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Support/Compiler.h#L206-L210), the condition only checked for the platform __WASM__ . Now Emscripten targets WebAssembly but doesn't imply the platform by default so the check isn't complete to define LLVM_ABI.

>From ed4c4946bdcce63eda9b2ef9072210197202c3a6 Mon Sep 17 00:00:00 2001
From: anutosh491 <andersonbhat491 at gmail.com>
Date: Mon, 17 Mar 2025 13:22:49 +0530
Subject: [PATCH] Define LLVM_ABI and CLANG_ABI for __EMSCRIPTEN__ builds

---
 clang/include/clang/Support/Compiler.h | 2 +-
 llvm/include/llvm/Support/Compiler.h   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Support/Compiler.h b/clang/include/clang/Support/Compiler.h
index 13582b899dc2a..5a74f8e3b6723 100644
--- a/clang/include/clang/Support/Compiler.h
+++ b/clang/include/clang/Support/Compiler.h
@@ -54,7 +54,7 @@
 #define CLANG_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_EXPORT_TEMPLATE
-#elif defined(__MACH__) || defined(__WASM__)
+#elif defined(__MACH__) || defined(__WASM__) || defined(__EMSCRIPTEN__)
 #define CLANG_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_TEMPLATE_ABI
 #define CLANG_EXPORT_TEMPLATE
diff --git a/llvm/include/llvm/Support/Compiler.h b/llvm/include/llvm/Support/Compiler.h
index f9c57b89f1f03..dc8b5389069eb 100644
--- a/llvm/include/llvm/Support/Compiler.h
+++ b/llvm/include/llvm/Support/Compiler.h
@@ -203,7 +203,7 @@
 #define LLVM_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define LLVM_EXPORT_TEMPLATE
 #define LLVM_ABI_EXPORT LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
-#elif defined(__MACH__) || defined(__WASM__)
+#elif defined(__MACH__) || defined(__WASM__) || defined(__EMSCRIPTEN__)
 #define LLVM_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define LLVM_TEMPLATE_ABI
 #define LLVM_EXPORT_TEMPLATE