These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

When we mark a module visible, we normally mark all of its non-explicit
submodules and other exports as visible. However, when we first enter a
submodule we should not make them visible to the submodule itself until
they are actually imported. Marking exports visible before import would
cause bizarre behaviour with local submodule visibility, because it
happened before we discovered the submodule's transitive imports and
could fail to make them visible in the parent module depending on
whether the submodules involved were explicitly defined (module X) or
implicitly defined from an umbrella (module *).

rdar://136524433

This patch avoids eagerly populating the submodule index on `Module`
construction. The `StringMap` allocation shows up in my profiles of
`clang-scan-deps`, while the index is not necessary most of the time. We
still construct it on-demand.

Moreover, this patch avoids performing qualified submodule lookup in
`ASTReader` whenever we're serializing a module graph whose top-level
module is unknown. This is pointless, since that's guaranteed to never
find any existing submodules anyway.

This speeds up `clang-scan-deps` by ~0.5% on my workload.

This patch shrinks the size of the `Module` class from 2112B to 1624B. I
wasn't able to get a good data on the actual impact on memory usage, but
given my `clang-scan-deps` workload at hand (with tens of thousands of
instances), I think there should be some win here. This also speeds up
my benchmark by under 0.1%.

In `clang-scan-deps`, we're creating lots of `Module` instances.
Allocating them all in a bump-pointer allocator reduces the number of
retired instructions by 1-1.5% on my workload.

Don't call raw_string_ostream::flush(), which is essentially a no-op.
As specified in the docs, raw_string_ostream is always unbuffered

`Module::Requirement` was defined as a `std::pair<std::string, bool>`.
This required a comment to explain what the data members mean and makes
the usage harder to understand. Replace this with a struct with two
members, `FeatureName` and `RequiredState`.

---------

Co-authored-by: cor3ntin <corentinjabot@gmail.com>

Co-authored-by: Alex Lorenz <arphaman@gmail.com>