Since commit 7cd65c97 (Add CMAKE_SYSROOT variable to set --sysroot
when cross compiling., 2013-04-13, v3.0.0-rc1~342^2) we have prefixed
the value of `CMAKE_SYSROOT` to implicit include directories.  This was
done because we hard-coded `/usr/include` as an implicit include
directory without accounting for the sysroot.  Instead we should prefix
the hard-coded paths when they are constructed.  Update the
`Platform/UnixPaths` module to do this as `Platform/Darwin` already
does.

Since commit 5990ecb7 (Compute implicit include directories from
compiler output, 2018-12-07, v3.14.0-rc1~108^2) the values of the
`CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES` variables are computed from
a real compiler invocation so they already account for the sysroot
prefix.  In commit 6fc33829 (Update logic for sysroot in detected
implicit include directories, 2019-02-13, v3.14.0-rc2~6^2) we attempted
to apply the prefix conditionally, but that is incorrect because the
compiler's real implicit include directories are not all under the
sysroot.  Instead assume that all implicit include directories in
`CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES` already have the sysroot
prefix if needed.  Code that constructs the value must be responsible
for that because it is the only place that knows.