Add helpers for finding any recent MSVC version.
Add helpers for finding any recent MSVC version.

Generalise findMsvc2017InstallDir() to findMsvcInstallDir()
to remove the hard-coded version check.

Also generalise getVisualStudio2107Compiler() to
getVisualStudioCompiler().

This should make it easier to have build systems find and
use more recent versions of Visual Studio.

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