Identified with misc-include-cleaner.

This reverts commit 0d288e5b0ccf217e41944ad4fd8772d8ae45daa1.

Breaks the build.

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

Identified with readability-qualified-auto.

MaterializationUnit::Interface holds the values that make up the interface
(for ORC's purposes) of a materialization unit: the symbol flags map and
initializer symbol.

Having a type for this will make functions that build materializer interfaces
more readable and maintainable.

The getPerDylibResources method may be called concurrently from multiple
threads, so we need to protect access to the underlying map.

Possible for fix https://llvm.org/PR51064

At most these use the StringRef/Twine wrappers and don't have any implicit uses of std::string.

Move the include down to any cpp implementation where std::string is actually used.