Change the linking of icu in libswiftCore to use icu-config instead of simply adding via an -l flag in order to capture the correct path

[lanza]

Using -licuXYZ on the command line selects whatever version shows up first in the search path. In the case of CentOS we have /usr/lib/libicuXYZ.so.50 being chosen which is not compatible with Swift. Using icu-config allows the user installed icu variants to take priority.

Requires
#14734
#14728

[lanza]

@compnerd

[jrose-apple]

This is a CMake file, not a shell script.

[compnerd]

ICU_UC ICU_I18N should be setup properly by the user. This is setup in build-script-impl IIRC. We shouldn't be using unix specific shell constructs in CMake.