First, more recent versions of Clang do no longer have trailing info in
parentheses, so the detection would fail. Since we're not interested
in this information, we just ignore it.
Then we should not rely on Clang being installed in the default program
folder. Instead we look up where the first clang.exe can be found, and
assume that it is located in a bin/ folder in the installation path.
From there we construct the library path, which is
`lib\clang\<ver>\lib\windows` where `<ver>` is either the full version
(older Clang) or only the major version. Note that this is the case
for stand-alone LLVM installations as well as Visual Studio supplied
ones.
Finally, we clean up by improving the error messages, and removing the
duplicate clang version detection in `add_asan_opts()`.
While we're at it, we also apply a cosmetic improvement to avoid
(trailing) whitespace in the compiler name (e.g. shown by `-v`).
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