[libc++] Fix modules build - Rework __refstring definition 

Summary:
`__libcpp_refstring` currently has two different definitions. First there is the complete definition in `<__refstring>` but there is also a second in  `<stdexcept>`.  The historical reason for this split is because both libc++ and libc++abi need to see the inline definitions of __libcpp_refstrings methods, but the `<stdexcept>` header doesn't.  However this is an ODR violation and breaks the modules build.

This patch fixes the issue by creating a single class definition in `<stdexcept>` and changing `<__refstring>` to contain only the inline method definitions. This way both `libcxx/src/stdexcept.cpp` and `libcxxabi/src/stdexcept.cpp` see the same declaration in `<stdexcept>` and definitions in `<__refstring>`

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25603

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@285100 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed