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

This patch implements the parsing and sema support for the OpenMP
'from'-clause with potential user-defined mappers attached.
User-defined mappers are a new feature in OpenMP 5.0. A 'from'-clause
can have an explicit or implicit associated mapper, which instructs the
compiler to generate and use customized mapping functions. 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 update from(mapper(id): ss) // use the mapper with name 'id' to map ss from device

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

Differential Revision: https://reviews.llvm.org/D58638

llvm-svn: 354817