[ASan] Use metadata to pass source-level information from Clang to ASan.
Instead of creating global variables for source locations and global names,
just create metadata nodes and strings. They will be transformed into actual
globals in the instrumentation pass (if necessary). This approach is more
- we don't have to ensure that our custom globals survive all the optimizations
- if globals are discarded for some reason, we will simply ignore metadata for them and won't have to erase corresponding globals
- metadata for source locations can be reused for other purposes: e.g. we may attach source location metadata to alloca instructions and provide better descriptions for stack variables in ASan error reports.
No functionality change.