Module: Don't rename in getOrInsertFunction()

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.

After some discussing with Rafael on the list, it looks like it's
undesired behaviour.  If a consumer actually *needs* this behaviour, we
should add new API with a more explicit name.

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

<rdar://problem/16165191>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203513 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Module.cpp

@@ -104,16 +104,6 @@ Constant *Module::getOrInsertFunction(StringRef Name,
     return New;                    // Return the new prototype.
   }
 
-  // Okay, the function exists.  Does it have externally visible linkage?
-  if (F->hasLocalLinkage()) {
-    // Clear the function's name.
-    F->setName("");
-    // Retry, now there won't be a conflict.
-    Constant *NewF = getOrInsertFunction(Name, Ty);
-    F->setName(Name);
-    return NewF;
-  }
-
   // If the function exists but has the wrong type, return a bitcast to the
   // right type.
   if (F->getType() != PointerType::getUnqual(Ty))