(Re-landing) Expose a TargetMachine::getTargetTransformInfo function

Re-land r321234.  It had to be reverted because it broke the shared
library build.  The shared library build broke because there was a
missing LLVMBuild dependency from lib/Passes (which calls
TargetMachine::getTargetIRAnalysis) to lib/Target.  As far as I can
tell, this problem was always there but was somehow masked
before (perhaps because TargetMachine::getTargetIRAnalysis was a
virtual function).

Original commit message:

This makes the TargetMachine interface a bit simpler.  We still need
the std::function in TargetIRAnalysis to avoid having to add a
dependency from Analysis to Target.

See discussion:
http://lists.llvm.org/pipermail/llvm-dev/2017-December/119749.html

I avoided adding all of the backend owners to this review since the
change is simple, but let me know if you feel differently about this.

Reviewers: echristo, MatzeB, hfinkel

Reviewed By: hfinkel

Subscribers: jholewinski, jfb, arsenm, dschuff, mcrosier, sdardis, nemanjai, nhaehnle, javed.absar, sbc100, jgravelle-google, aheejin, kbarton, llvm-commits

Differential Revision: https://reviews.llvm.org/D41464

llvm-svn: 321375