[OpenMP 5.0] Parsing/sema support for map clause with mapper modifier.

This patch implements the parsing and sema support for OpenMP map
clauses with potential user-defined mapper attached. User defined mapper
is a new feature in OpenMP 5.0. A map clause can have an explicit or
implicit associated mapper, which instructs the compiler to generate
extra data mapping. An example is shown below:

    struct S { int len; int *d; };
    #pragma omp declare mapper(id: struct S s) map(s, s.d[0:s.len])
    struct S ss;
    #pragma omp target map(mapper(id) tofrom: ss) // use the mapper with name 'id' to map ss

Contributed-by: Lingda Li <lildmh@gmail.com>

Differential Revision: https://reviews.llvm.org/D58074

llvm-svn: 354347