Calling AST node "Length" breaks compilation

How to reproduce

Create an example project:

create-project.py repro

Change ExampleNode to Length:

$ sed -ie 's/ExampleNode/Length/' repro/language/parser.py
$ cd repro && python3 ./manage.py make
[...]
Compile
   [Ada]          parse.adb
parse.adb:137:44: "Length" is not visible
parse.adb:137:44: multiple use clauses cause hiding
parse.adb:137:44: hidden declaration at libreprolang-analysis.ads:88
parse.adb:137:44: hidden declaration at a-strunb.ads:92

   compilation of parse.adb failed

gprbuild: *** compilation phase failed
Build failed: error while running gprbuild -p -j8 -P/home/senier/tmp/langkit/length_issue/repro/build/src/mains.gpr -XBUILD_MODE=dev -XLIBRARY_TYPE=relocatable -XGPR_BUILD=relocatable -XXMLADA_BUILD=relocatable parse.adb:
    Command '['gprbuild', '-p', '-j8', '-P/home/senier/tmp/langkit/length_issue/repro/build/src/mains.gpr', '-XBUILD_MODE=dev', '-XLIBRARY_TYPE=relocatable', '-XGPR_BUILD=relocatable', '-XXMLADA_BUILD=relocatable', 'parse.adb']' returned non-zero exit status 4.

Expected behavior

Either reject Length as AST node name or (preferably) allow it's use in the parser by fully qualifying Ada.Strings.Unbounded.Length (maybe use clauses should be avoided in generated code alltogether).

Versions used

GNAT: Community 2020
Langkit: master (45df941275a0844fafc960cfc6b775b1108f42f3)

Reproduce the failure with create-project.py, then inspect repro/language/parser.py and the generated parse.adb around the reported line. Check the declarations in libreprolang-analysis.ads and the conflicting Ada.Strings.Unbounded declaration; done means the Length AST-node case is either rejected clearly or compiles without the name collision.