Add ABI option to remove recently inlined __shared_count functions from the library.

In order to allow inlining of previously out-of-line functions without an ABI break
libc++ provides legacy definitions in the dylib that old programs can
continue to use. Unfortunatly Windows link.exe detects this hack and diagnoses the duplicate
definitions.

This patch disable the duplicate definitions on Windows by adding an ABI option
which disables all "legacy out-of-line symbols"

llvm-svn: 292190