Add a new Cygwin toolchain that just goes through the motions to
initialize the Generic_GCC base properly. This allows removing some old,
almost certainly wrong hard-coded paths from Lex/InitHeaderSearch.cpp.
MSYS2 (GCC triple (arch)-pc-msys) is a fork of Cygwin (GCC triple
(arch)-pc-cygwin), and this driver can be used for either.
Add a simple test for this driver.
