Clang 8 or 9 seems to have changed from EXE to exe. Allow both
capitalizations (which is not important in Windows) as a workaround.
Maybe in the future we can remove the uppercase option.
Commit to a command line option spelling so that build systems can
start testing it. I deliberately picked one of the longer names we
were considering because we can always decide to add a shorter alias,
but can't decide a shorter name was too generic.
Like the other supplementary output flags,
-emit-parseable-module-interface-path will emit a .swiftinterface file
to a particular path, while -emit-parseable-module-interface will put
it next to the main output (the one specified with -o).
rdar://problem/43776945
