CMake: Add QTC_USE_LLVM_LINKER option

llvm-link is faster on macOS than ld64. llvm-link can also be used with
MinGW GCC's ld which is very slow in debug mode.

Change-Id: Ie519f3111e05cfc7dffab2676108a0d53ebe08b2
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

CMakeLists.txt

@@ -30,6 +30,7 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
 qtc_handle_compiler_cache_support()
+qtc_handle_llvm_linker()
 
 option(BUILD_LINK_WITH_QT "Link with Qt from the parent Qt Creator" OFF)
 qtc_link_with_qt()

cmake/QtCreatorAPI.cmake

@@ -46,6 +46,7 @@ option(QTC_SEPARATE_DEBUG_INFO "Extract debug information from binary files." OF
 option(WITH_SCCACHE_SUPPORT "Enables support for building with SCCACHE and separate debug info with MSVC, which SCCACHE normally doesn't support." OFF)
 option(WITH_CCACHE_SUPPORT "Enables support for building with CCACHE and separate debug info with MSVC, which CCACHE normally doesn't support." "${ENV_WITH_CCACHE_SUPPORT}")
 option(QTC_STATIC_BUILD "Builds libraries and plugins as static libraries" OFF)
+option(QTC_USE_LLVM_LINKER "Uses llvm-link linker if found." OFF)
 
 # If we provide a list of plugins, executables, libraries, then the BUILD_<type>_BY_DEFAULT will be set to OFF
 # and for every element we set BUILD_<type>_<elment> to ON

cmake/QtCreatorAPIInternal.cmake

@@ -141,6 +141,15 @@ function(qtc_handle_compiler_cache_support)
   endif()
 endfunction()
 
+function(qtc_handle_llvm_linker)
+  if (QTC_USE_LLVM_LINKER)
+    find_program(LLVM_LINK_PROGRAM llvm-link)
+    if(LLVM_LINK_PROGRAM)
+      set(CMAKE_LINKER "${LLVM_LINK_PROGRAM}" CACHE STRING "LLVM linker" FORCE)
+    endif()
+  endif()
+endfunction()
+
 function(qtc_link_with_qt)
   # When building with Qt Creator 4.15+ do the "Link with Qt..." automatically
   if (BUILD_LINK_WITH_QT AND DEFINED CMAKE_PROJECT_INCLUDE_BEFORE)