[NativePDB] Reconstruct function declarations from debug info.

Previously we would create an lldb::Function object for each function
parsed, but we would not add these to the clang AST. This is a first
step towards getting local variable support working, as we first need an
AST decl so that when we create local variable entries, they have the
proper DeclContext.

Differential Revision: https://reviews.llvm.org/D55384

llvm-svn: 348631