swift234

Revert "CMake: don't hide symbol names in bitcode for debuggability"

This reverts commit r31313.

This change caused an unacceptable dylib size increase:
- watchOS core dylib size increased from 13 Mb to 25 Mb,
- iOS dylib size increased 40 Mb to 76 Mb.

The size change is due to change of the bitcode section size.  It can be
attributed to:
- actual symbol strings that we have started to include;
- full debug info.  Symbol obfuscation was also changing full debug info
  in LLVM metadata into line tables.

rdar://22330874, again.

Swift SVN r31473