[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
flexible:
1) we don't have to ensure that our custom globals survive all the optimizations
2) if globals are discarded for some reason, we will simply ignore metadata for them
   and won't have to erase corresponding globals
3) 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.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@214604 91177308-0d34-0410-b5e6-96231b3b80d8
In the case of mangling collisions, make an attempt to note both definitions
involved.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@214606 91177308-0d34-0410-b5e6-96231b3b80d8