[Support] Lazy load of dbghlp.dll on Windows

This patch changes linkage with dbghlp.dll for clang from static (at load time)
to on demand (at the first use of required functions). Clang uses dbghlp.dll
only in minor use-cases. First of all in case of crash and in case of plugin load.
The dbghlp.dll library can be absent on system. In this case clang will fail
to load. With lazy load of dbghlp.dll clang can work even if dbghlp.dll
is not available.

Differential Revision: http://reviews.llvm.org/D10737

llvm-svn: 241271