llvm-lto: add a -thinlto-module-id that enables to force the Module identifier.

ThinLTO is using the Module Identifier to find the corresponding entry
in the index. However when reproducing part of the flow from temporary
files generated from the linker, you'd like to process a file and
force llvm-lto to use another module identifier than the current
filename. The alternative would be to tweak the index, which would be
more involved.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 268643