Find versioned versions of LLVM tools

Since it has become more widely know that the LLVM developers often make breaking changes to the IR language between releases, some Linu distributions (eg Debain and hence Ubuntu) have started doing multi-version LLVM installs.

For instance on Debian, the llvm-3.6 programs are installed in /usr/lib/llvm-3.6/bin as llc and opt.

I propose that the configure.ac and aclocal.m4 configuration goop be modified to look for llc and opt in these versioned /usr/lib/llvm-X.Y locations. I'm also willing to do the hard yards on implementing this if people think its a good idea.

This can be seen as a step along the way to more stringent LLVM versioning requirements.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (LLVM)
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