[flang][Driver] Enable FLANG_DEFAULT_LINKER

The default linker could be changed by a CMake variable CLANG_DEFAULT_LINKER before.
However, it also changes the default linker invoked by clang.
This patch distincts the default linkers of clang and flang.

Note that FLANG_DEFAULT_LINKER will be the same as CLANG_DEFAULT_LINKER
unless it is defined explicitly.
That means this patch does not affect the current behavior.

Fixes #73153

Author: parabola94

flang/CMakeLists.txt

@@ -317,7 +317,7 @@ if (NOT ENABLE_LINKER_BUILD_ID)
   set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 endif()
 
-set(FLANG_DEFAULT_LINKER "" CACHE STRING
+set(FLANG_DEFAULT_LINKER ${CLANG_DEFAULT_LINKER} CACHE STRING
   "Default linker to use (linker name or absolute path, empty for platform default)")
 
 set(FLANG_DEFAULT_RTLIB "" CACHE STRING

flang/include/flang/Config/config.h.cmake

@@ -1,10 +1,10 @@
-#===-- include/flang/Config/config.h.cmake ---------------------------------===#
-#
-# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-# See https://llvm.org/LICENSE.txt for license information.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-#
-#===------------------------------------------------------------------------===#
+//===-- include/flang/Config/config.h.cmake ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===------------------------------------------------------------------------===//
 
 /* This generated file is for internal use. Do not include it from headers. */
 
@@ -16,6 +16,8 @@
 
 #define FLANG_VERSION            "${FLANG_VERSION}"
 
+#define FLANG_DEFAULT_LINKER     "${FLANG_DEFAULT_LINKER}"
+
 #endif
 
 

flang/tools/flang-driver/driver.cpp

@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Driver/Driver.h"
+#include "flang/Config/config.h"
 #include "flang/Frontend/CompilerInvocation.h"
 #include "flang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Basic/Diagnostic.h"
@@ -138,6 +139,7 @@ int main(int argc, const char **argv) {
                                   llvm::sys::getDefaultTargetTriple(), diags,
                                   "flang LLVM compiler");
   theDriver.setTargetAndMode(targetandMode);
+  theDriver.setPreferredLinker(FLANG_DEFAULT_LINKER);
 #ifdef FLANG_RUNTIME_F128_MATH_LIB
   theDriver.setFlangF128MathLibrary(FLANG_RUNTIME_F128_MATH_LIB);
 #endif