[clang] use string tables for static diagnostic descriptions

Using a pointer for the description string in StaticDiagInfoRec causes
several problems:

1. We don't need to use a whole pointer to represent the string;
2. The use of pointers incurs runtime relocations for those pointers;
   the relocations take up space on disk and represent runtime overhead;
3. The need to relocate data implies that, on some platforms, the entire
   array containing StaticDiagInfoRecs cannot be shared between processes.

This patch changes the storage scheme for the diagnostic descriptions to
avoid these problems.  We instead generate (effectively) one large
string and then StaticDiagInfoRec conceptually holds offsets into the
string.  We elected to also move the storage of those offsets into a
separate array to further reduce the space required.

On x86-64 Linux, this change removes about 120KB of relocations and
moves about 60KB from the non-shareable .data.rel.ro section to
shareable .rodata.  (The array is about 80KB before this, but we
eliminated 4 bytes/entry by using offsets rather than pointers.)  We
actually reap this benefit twice, because these tables show up in both
libclang.so and libclang-cpp.so and we get the reduction in both places.

Differential Revision: https://reviews.llvm.org/D81865

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