[NativePDB] Support member function types in PdbAstBuilder

    Summary:
    This patch implements missing case in PdbAstBuilder::CreateType for
    LF_MFUNCTION. This is necessary, for example, in stack unwinding of struct
    methods.

    Reviewers: amccarth, aleksandr.urakov

    Reviewed By: amccarth

    Subscribers: abidh, teemperor, lldb-commits, leonid.mashinskiy

    Differential Revision: https://reviews.llvm.org/D61128

llvm-svn: 360569