[pseudo] Move pseudoparser from clang to clang-tools-extra

This should make clearer that:
 - it's not part of clang proper
 - there's no expectation to update it along with clang (beyond green tests)
 - clang should not depend on it

This is intended to be expose a library, so unlike other tools has a split
between include/ and lib/.

The main renames are:
  clang/lib/Tooling/Syntax/Pseudo/*           => clang-tools-extra/pseudo/lib/*
  clang/include/clang/Tooling/Syntax/Pseudo/* => clang-tools-extra/pseudo/include/clang-pseudo/*
  clang/tools/clang/pseudo/*                  => clang-tools-extra/pseudo/tool/*
  clang/test/Syntax/*                         => clang-tools-extra/pseudo/test/*
  clang/unittests/Tooling/Syntax/Pseudo/*     => clang-tools-extra/pseudo/unittests/*
  #include "clang/Tooling/Syntax/Pseudo/*"    => #include "clang-pseudo/*"
  namespace clang::syntax::pseudo             => namespace clang::pseudo
  check-clang                                 => check-clang-pseudo
  clangToolingSyntaxPseudo                    => clangPseudo
The clang-pseudo and ClangPseudoTests binaries are not renamed.

See discussion around:
https://discourse.llvm.org/t/rfc-a-c-pseudo-parser-for-tooling/59217/50

Differential Revision: https://reviews.llvm.org/D121233