[flang] CUDA Fortran - part 1/5: parsing

Begin upstreaming of CUDA Fortran support in LLVM Flang.

This first patch implements parsing for CUDA Fortran syntax,
including:
 - a new LanguageFeature enum value for CUDA Fortran
 - driver change to enable that feature for *.cuf and *.CUF source files
 - parse tree representation of CUDA Fortran syntax
 - dumping and unparsing of the parse tree
 - the actual parsers for CUDA Fortran syntax
 - prescanning support for !@CUF and !$CUF
 - basic sanity testing via unparsing and parse tree dumps

... along with any minimized changes elsewhere to make these
work, mostly no-op cases in common::visitors instances in
semantics and lowering to allow them to compile in the face
of new types in variant<> instances in the parse tree.

Because CUDA Fortran allows the kernel launch chevron syntax
("call foo<<<blocks, threads>>>()") only on CALL statements and
not on function references, the parse tree nodes for CallStmt,
FunctionReference, and their shared Call were rearranged a bit;
this caused a fair amount of one-line changes in many files.

More patches will follow that implement CUDA Fortran in the symbol
table and name resolution, and then semantic checking.

Differential Revision: https://reviews.llvm.org/D150159