Convert a location information from PDB to a DWARF expression

The current version of SymbolFilePDB::ParseVariableForPDBData function
always initializes variables with an empty location. This patch adds the
converter of a location information from PDB to a DWARF expression, so
it becomes possible to watch values of variables of primitive data
types. At the moment the converter supports only Static, TLS, RegRel,
Enregistered and Constant PDB location types, but it seems that it's
enough for most cases. There are still some problems with retrieving
values of variables (e.g. we can't watch variables of composite types),
but they look not relevant to the conversion to DWARF.

Patch by: Aleksandr Urakov

Differential revision: https://reviews.llvm.org/D49018

llvm-svn: 336988