Expose QualType::getUnqualifiedType in libclang

The method is now wrapped by clang_getUnqualifiedType.

A declaration for clang_getUnqualifiedType was added to
clang-c/Index.h to expose it to user of the library.

An implementation for clang_getUnqualifiedType was introduced in
CXType.cpp that wraps the equivalent method of the underlying
QualType of a CXType.

An export symbol was added to libclang.map under the new version entry
LLVM_16.

A test was added to LibclangTest.cpp that tests the removal of
qualifiers for some CXTypes.

Differential Revision: https://reviews.llvm.org/D132749