[PATCH] libLTO: Allow linker to choose context of modules and codegen

[Duncan P. N. Exon Smith]

The attached patches add API for specifying which `LLVMContext` each
`lto_module_t` and `lto_code_gen_t` is in.  combined.patch has the
combine diff.

This enables the following flow:

    for (auto &File : Files) {
      lto_module_t M = lto_module_create_in_local_context(File...);
      querySymbols(M);
      lto_module_dispose(M);
    }

    lto_code_gen_t CG = lto_codegen_create_in_local_context();
    for (auto &File : FilesToLink) {
      lto_module_t M = lto_module_create_in_codegen_context(File..., CG);
      lto_codegen_add_module(CG, M);
      lto_module_dispose(M);
    }
    lto_codegen_compile(CG);
    lto_codegen_write_merged_modules(CG, ...);
    lto_codegen_dispose(CG);

This flow has a few benefits.

  - Only one module (two if you count the combined module in the code
    generator) is in memory at a time.

  - Metadata (and constants) from files that are parsed to query symbols
    but not linked into the code generator don't pollute the global
    context.

  - The first for loop can be parallelized, since each module is in its
    own context.

  - When the code generator is disposed, the memory from LTO gets freed.

I'm not really sure how best to test this.  Let me know if you have
any ideas.

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