Some functions like memchr are defined both in libc++ and libc.
Including both would result in ambiguous references at the call sites.
This is worked around by an attribute that tells the compiler to prefer
one overload over the others. This attribute was not interpreted by
Swift. As a result, importing both libc and libc++ and calling such
functions resulted in compilation errors due to ambiguous overloads.
This PR modifies the lookup logic to exclude the non-preferred Clang
functions from the overload set whenever a preferred overload is
available.
rdar://152192945
This fixes a compiler bug that got exposed by f11abac652.
If a C++ type is declared in a nested Clang submodule, Swift was emitting errors that look like:
```
Type alias 'string' is not available due to missing import of defining module 'fwd’
```
rdar://146899125
This makes sure that when Swift is generating a `.swiftinterface` file for a Swift module with a dependency on C++ module, `-enable-experimental-cxx-interop` is emitted under `// swift-module-flags:`.
The module interface might refer to C++ symbols which are not available in Swift without C++ interop enabled. This caused a build error for Swift LLVM bindings during `verify-module-interface` stage: Swift tried to import the module interface and couldn't find C++ stdlib headers because C++ interop was disabled.
