[NativePDB] Get LLDB types from PDB function types.

This adds basic support for getting function signature types
into LLDB's type system, including into clang's AST.  There are
a few edge cases which are not correctly handled, mostly dealing
with nested classes, but this isn't specific to functions and
apply equally to variable types.  Note that no attempt has been
made yet to deal with member function types, which will happen
in subsequent patches.

Differential Revision: https://reviews.llvm.org/D53951

llvm-svn: 345848