[IR] make -stack-alignment= into a module attr

Authored by nickdesaulniers on Jun 8 2021, 8:22 AM.


[IR] make -stack-alignment= into a module attr

Similar to D102742, specifying the stack alignment via CodegenOpts means
that this flag gets dropped during LTO, unless the command line is
re-specified as a plugin opt. Instead, encode this information as a
module level attribute so that we don't have to expose this llvm
internal flag when linking the Linux kernel with LTO.

Looks like external dependencies might need a fix:

Link: https://github.com/ClangBuiltLinux/linux/issues/1377

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D103048