Merging r195556:
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r195556 | akirtzidis | 2013-11-23 10:41:35 -0800 (Sat, 23 Nov 2013) | 3 lines

[CodeGen] If there is a function definition with duplicate mangled name, emit an error instead of asserting.

rdar://15522601 & http://llvm.org/PR18031
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_34@195623 91177308-0d34-0410-b5e6-96231b3b80d8