[MSAN] Separate id ptr from constant string for variable names used in track origins.

The goal is to reduce the size of the MSAN with track origins binary, by making
the variable name locations constant which will allow the linker to compress
them.

Follows: https://reviews.llvm.org/D131415

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D131631