Add a key method to Sema to optimize debug info size
It turns out that the debug info describing the Sema class is an
appreciable percentage of the total object file size of objects in Sema.
By adding a key function, clang is able to optimize the debug info size
by emitting a forward declaration in TUs that do not define the key
On Windows, with clang-cl, these are the total sizes of object files in
Sema before and after this change, compiling with optimizations and
before: 335,012 KB after: 278,116 KB delta: -56,896 KB percent: -17.0%
The effect on link time was negligible, despite having ~56MB less input.
On Linux, with clang, these are the same sizes using DWARF -g and
before: 603,756 KB after: 515,340 KB delta: -88,416 KB percent: -14.6%
I didn't use type units, DWARF-5, fission, or any other special flags.
Reviewed By: thakis
Differential Revision: https://reviews.llvm.org/D70340