Add clang_CXXMethod_isExplicit to libclang

The new method is a wrapper of `CXXConstructorDecl::isExplicit` and
`CXXConversionDecl::isExplicit`, allowing the user to recognize whether
the declaration pointed to by a cursor was marked with the explicit
specifier.

An export for the function, together with its documentation, was added
to "clang/include/clang-c/Index.h" with an implementation provided in
"clang/tools/libclang/CIndex.cpp".

The implementation is based on similar `clang_CXXMethod`
implementations, returning a falsy unsigned value when the cursor is not
a declaration, is not a declaration for a constructor or conversion
function or is not a relevant declaration that was marked with the
`explicit` specifier.

The new symbol was added to "clang/tools/libclang/libclang.map" to be
exported, under the LLVM16 tag.

"clang/tools/c-index-test/c-index-test.c" was modified to print a
specific tag, "(explicit)", for cursors that are recognized by
`clang_CXXMethod_isExplicit`.

Two new regression files, "ex...