[Assignment Tracking][2/*] Add flags to enable Assignment Tracking

The Assignment Tracking debug-info feature is outlined in this RFC:

https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir

Enable in clang: -Xclang -fexperimental-assignment-tracking
Enable in llvm tools: -experimental-assignment-tracking

When assignment tracking is enabled in clang it will pass on the flag to enable
the feature in lllvm. It's undefined behaviour to read IR that contains
assignment tracking metadata without specifying the feature flags.

Tests will come with later patches that add assignment tracking features.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D132221