[libc++] Run clang-tidy only once per header (#121436)

There doesn't seem to be much of a reason to run clang-tidy twice per
headers, and running it only once makes the test a few seconds faster.

Author: philnik777

libcxx/.clang-tidy

@@ -5,6 +5,8 @@ Checks: >
   bugprone-stringview-nullptr,
   bugprone-use-after-move,
 
+  libcpp-*,
+
   llvm-include-order,
   llvm-namespace-comment,
 

libcxx/test/libcxx/clang_tidy.gen.py

@@ -33,8 +33,7 @@
 {lit_header_undeprecations.get(header, '')}
 
 // TODO: run clang-tidy with modules enabled once they are supported
-// RUN: %{{clang-tidy}} %s --warnings-as-errors=* -header-filter=.* --checks='-*,libcpp-*' --load=%{{test-tools-dir}}/clang_tidy_checks/libcxx-tidy.plugin -- %{{compile_flags}} -fno-modules
-// RUN: %{{clang-tidy}} %s --warnings-as-errors=* -header-filter=.* --config-file=%{{libcxx-dir}}/.clang-tidy -- -Wweak-vtables %{{compile_flags}} -fno-modules
+// RUN: %{{clang-tidy}} %s --warnings-as-errors=* -header-filter=.* --config-file=%{{libcxx-dir}}/.clang-tidy --load=%{{test-tools-dir}}/clang_tidy_checks/libcxx-tidy.plugin -- -Wweak-vtables %{{compile_flags}} -fno-modules
 
 #include <{header}>
 """)