Misc enhancements to LTO:

1. Add some helper classes for partitions. They are designed in a
     way such that the top-level LTO driver will not see much difference 
     with or without partitioning.

  2. Introduce work-dir. Now all intermediate files generated during 
     LTO phases will be saved under work-dir. User can specify the workdir
     via -lto-workdir=/path/to/dir. By default the work-dir will be 
     erased before linker exit. To keep the workdir, do -lto-keep, or -lto-keep=1.

    TODO: Erase the workdir, if the linker exit prematurely.  
      We are currently not able to remove directory on signal. The support 
      routines simply ignore directory.

  3. Add one new API lto_codegen_get_files_need_remove().
     Linker and LTO plugin will communicate via this API about which files
    (including directories) need to removed before linker exit.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188188 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm-c/lto.h

@@ -284,6 +284,18 @@ lto_codegen_compile(lto_code_gen_t cg, size_t* length);
 extern bool
 lto_codegen_compile_to_file(lto_code_gen_t cg, const char** name);
 
+/**
+ * Get intermediate files that need to be removed before linker exit. Upon
+ * return, the paths of the files need to be removed is written to "paths". The
+ * paths are separated by a single '\0', and the last path is ended by double
+ * '\0's. A file could be a directory; in this case, the entire directory needs
+ * to be removed recursively.
+ *
+ * It is only necessary to call this function after \p lto_codegen_compile was
+ * successfully called.
+ */
+extern void
+lto_codegen_get_files_need_remove(lto_code_gen_t cg, const char **paths);
 
 /**
  * Sets options to help debug codegen bugs.