[PDB] Support executables and source/line info.

Previously DebugInfoPDB could only load data for a PDB given a
path to the PDB.  It could not open an EXE and find the matching
PDB and verify it matched, etc.  This patch adds support for that
so that we can simply load debug information for a PDB directly.

Additionally, this patch extends DebugInfoPDB to support getting
source and line information for symbols.

llvm-svn: 235237