Please backport "[cmake] Include GNUInstallDirs before using variables defined by it." to 19.x

[mgorny]

A trivial CMake fix: 5bbd598

commit 5bbd5984306ab0bdd89a2e81cd4965e5ae51c3fb
Author:     Vassil Vassilev <v.g.vassilev@gmail.com>
AuthorDate: 2024-08-22 13:04:33 +0200
Commit:     GitHub <noreply@github.com>
CommitDate: 2024-08-22 13:04:33 +0200

    [cmake] Include GNUInstallDirs before using variables defined by it. (#83807)
    
    This fixes an odd problem with the regex when `CMAKE_INSTALL_LIBDIR` is
    not defined:
    
    `string sub-command REGEX, mode REPLACE: regex "$" matched an empty
    string.`
    
    Fixes llvm/llvm-project#83802

[mgorny]

/cherry-pick 5bbd598

[llvmbot]

/pull-request #121410