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[ThinLTO]LTO]Legacy] Fix dependent libraries support by adding querying of the…

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[ThinLTO]LTO]Legacy] Fix dependent libraries support by adding querying of the IRSymtab

Dependent libraries support for the legacy api was committed in a
broken state (see: https://reviews.llvm.org/D60274). This was missed
due to the painful nature of having to integrate the changes into a
linker in order to test. This change implements support for dependent
libraries in the legacy LTO api:

  • I have removed the current api function, which returns a single

string, and added functions to access each dependent library
specifier individually.

  • To reduce the testing pain, I have made the api functions as thin as

possible to maximize coverage from llvm-lto.

  • When doing ThinLTO the system linker will load the modules lazily

when scanning the input files. Unfortunately, when modules are
lazily loaded there is no access to module level named metadata. To
fix this I have added api functions that allow querying the IRSymtab
for the dependent libraries. I hope to expand the api in the future
so that, eventually, all the information needed by a client linker
during scan can be retrieved from the IRSymtab.

Differential Revision: https://reviews.llvm.org/D62935

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