[flang][docs][nfc] Refine FlangOptionsDocs.td

Currently, FlangOptionsDocs.td doesn't specify `ExcludedFlags` which
means that in the generated documentation file we expose flags that:
  * we don't necessarily won't to advertise to our users (e.g. hidden flags), or
  * are not supported altogether (e.g. CL options).
This patch defines `ExcludeFlags` to fix that. The definition of
`ExcludeFlags` was copied from Clang so that LLVM frontends have
consistent documentation.

It might be a bit counter-intuitive that IncludeFlags alone is not
sufficient here. However, the current logic in ClangOptionDocEmitter.cpp
will parse IncludeFlags and print all options that contains one of the
included flags, as well as their aliases. So, for example, for -fopenmp
(which is a supported Flang option), one would also get /fopenmp (i.e.
CL mode equivalent for -fopenmp). By adding ExcludeFlags, we make sure
that such aliases are excluded.

I've also taken the liberty and moved FlangOptionsDocs.td. Originally it
was located in Flang's "flang/include" directory, but there shouldn't be
any implementation/documentation files there. Instead, I'm moving it to
the "flang/docs" directory.

Differential Revision: https://reviews.llvm.org/D130558

flang/docs/CMakeLists.txt

@@ -97,6 +97,7 @@ function (gen_rst_file_from_td output_file td_option source docs_target)
   endif()
   get_filename_component(TABLEGEN_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/${source}" DIRECTORY)
   list(APPEND LLVM_TABLEGEN_FLAGS "-I${TABLEGEN_INCLUDE_DIR}")
+  list(APPEND LLVM_TABLEGEN_FLAGS "-I${CMAKE_CURRENT_SOURCE_DIR}/../../clang/include/clang/Driver/")
   clang_tablegen(Source/${output_file} ${td_option} SOURCE ${source} TARGET "gen-${output_file}")
   add_dependencies(${docs_target} "gen-${output_file}")
 endfunction()
@@ -126,7 +127,7 @@ if (LLVM_ENABLE_SPHINX)
 
       # CLANG_TABLEGEN_EXE variable needs to be set for clang_tablegen to run without error
       set(CLANG_TABLEGEN_EXE clang-tblgen)
-      gen_rst_file_from_td(FlangCommandLineReference.rst -gen-opt-docs ../include/flang/FlangOptionsDocs.td docs-flang-html)
+      gen_rst_file_from_td(FlangCommandLineReference.rst -gen-opt-docs FlangOptionsDocs.td docs-flang-html)
     endif()
     if (${SPHINX_OUTPUT_MAN})
       add_sphinx_target(man flang)

flang/docs/FlangOptionsDocs.td

@@ -26,10 +26,11 @@ Introduction
 
   string Program = "flang";
 
-  list<string> ExcludedFlags = [];
+  list<string> ExcludedFlags = ["HelpHidden", "NoDriverOption",
+                                "CLOption", "Unsupported", "Ignored"];
   list<string> IncludedFlags = ["FlangOption"];
 
 }
 
 
-include "../../../clang/include/clang/Driver/Options.td"
+include "Options.td"