[PATCH] Module: Use existing library functions

[Duncan P. N. Exon Smith]

During LTO, user-supplied definitions of C library functions often
exist.  -instcombine uses Module::getOrInsertFunction() to get a handle
on library functions (e.g., @puts, when optimizing @printf).

Previously, Module::getOrInsertFunction() would rename any matching
functions with local linkage, and create a new declaration.  In LTO,
this is the opposite of desired behaviour, as it skips by the
user-supplied version of the library function and creates a new
undefined reference which the linker often cannot resolve.

Removing that logic did not cause any tests to fail, and I couldn't
think of why it's *good* behaviour.  Maybe no one was using it?

I added two testcases: one specifically for the -instcombine behaviour
and one for the LTO flow that exposed it.

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