Improvements to finding LLVM's FileCheck

This patch adds a few improvements to how the build system finds
LLVM's FileCheck program.

* On Fedora, the system LLVM installs FileCheck in the "llvm"
  subdirectory of the LLVM libdir.  This patch teaches the build
  system to look there.

* This adds a configure option to specify which llvm-config executable
  to use.  This is handy on systems that can parallel install multiple
  versions of LLVM; for example I can now:

    ./configure --llvm-config=/bin/llvm-config-5.0-64

  ... to build against LLVM 5, rather than whatever the default
  llvm-config might be.

* Finally, this adds a configure- and config.toml- option to set the
  path to FileCheck.  This is handy when building against an LLVM
  where FileCheck was not installed.  This happens on compatibility
  installs of LLVM on Fedora.

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