[llvm] Use `GNUInstallDirs` to support custom installation dirs

This is the patch for LLVM proper in my series for adding GNUInstallDirs support in all project.

Additionally:

Create a new `CACHE STRING` variable, `LLVM_EXAMPLES_INSTALL_DIR`, to control where the examples are installed on analogy with the other variables.

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This patch supersedes D28234, which tried to do the same thing but hand-rolled without GNUInstallDirs.

This patch nearly reverts commit 3 0fc88bf1dc15a72e2d9809d28019d386b7a7cc0, which was a revert of a prior attempt."

(I had to add a space here or else Phabricator detects a reference cycle and won't let me do the form submit.)

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D100810