Moves UnicodeMap and related classes to core.
Also removes `CollectionUtilities`, `UOption`, and `ElapsedTimer`.
They will end up in UnicodeTools, CLDR, and CLDR respectively.
When building icu4c, it defaults to clang instead of gcc when the default compiler, cc / c++, is a symlink to gcc / g++.
This not the expected behavior when building C and C++ code.
It appears that this behavior was put in place originally for supporting C++11, which hopefully is no longer such a concern.
This PR adjusts the configure.ac for icu4c to prefer the cc and c++ compilers first.
