This is a follow up to #112015 and it reduces the unnecessary
duplication of source locations further.
We do not need to allocate source location space in the serialized PCMs
for module maps used only to find textual headers. Those module maps are
never referenced from anywhere in the serialized ASTs and are re-read in
other compilations.
This change should not affect correctness of Clang compilations or
clang-scan-deps in any way.
We do need the InputFile entry in the serialized AST because
clang-scan-deps relies on it. The previous patch introduced a mechanism
to do exactly that.
We have found that to finally remove any duplication of module maps we
use internally in our build system.
