build: honour LLVM_INSTALL_TOOLCHAIN_ONLY

This improves the integration with LLVM and the unified build.  The
LLVM_INSTALL_TOOLCHAIN_ONLY flag ensures that the the development libraries are
not installed.  Because this option is not defined by default, the beahavioural
changes here are not triggered by default.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -1433,11 +1433,13 @@ function(add_swift_host_library name)
     INSTALL_IN_COMPONENT "dev"
     )
 
-  swift_install_in_component(dev
-    TARGETS ${name}
-    ARCHIVE DESTINATION lib${LLVM_LIBDIR_SUFFIX}
-    LIBRARY DESTINATION lib${LLVM_LIBDIR_SUFFIX}
-    RUNTIME DESTINATION bin)
+  if(NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
+    swift_install_in_component(dev
+      TARGETS ${name}
+      ARCHIVE DESTINATION lib${LLVM_LIBDIR_SUFFIX}
+      LIBRARY DESTINATION lib${LLVM_LIBDIR_SUFFIX}
+      RUNTIME DESTINATION bin)
+  endif()
 
   swift_is_installing_component(dev is_installing)
   if(NOT is_installing)