Do version specific detection of LLVM tools (#10170).
The LLVM developers seem to make breaking changes in the LLVM IR language between major releases. As a consumer of the LLVM tools GHC now needs to be locked more tightly to a single version of the LLVM tools. GHC HEAD currently only supports LLVM version 3.6. This commit changes the configure script to look for `llc-3.6` and `opt-3.6` before looking for `llc` and `opt`. If the former are not found, but the later are, check that they actually are version 3.6. At the same time, when detecting known problems with the LLVM tools (ie #9439) test for it using the versions of the LLVM tools retrieved from the bootstrap compiler's settings file. Test Plan: Manual testing. Reviewers: thomie, rwbarton, nomeata, austin Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D745 GHC Trac Issues: #10170
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