[libc][docs] Fix docgen macro lookup for underscored headers (#194367)

While adding implementation status for nl_types.h, I noticed docgen
resolves it to nl-types.h instead of nl_types.h. As a result, headers
with underscores are not matched correctly and their implementation
status is not marked.

This patch fixes the handling of underscored header names in docgen so
they are processed consistently.

libc/include/llvm-libc-macros/CMakeLists.txt

@@ -371,7 +371,7 @@ add_macro_header(
 add_macro_header(
   nl_types_macros
   HDR
-    nl-types-macros.h
+    nl_types-macros.h
 )
 
 add_macro_header(

libc/include/nl_types.yaml

@@ -3,9 +3,9 @@ standards:
   - posix
 macros:
   - macro_name: NL_SETD
-    macro_header: nl-types-macros.h
+    macro_header: nl_types-macros.h
   - macro_name: NL_CAT_LOCALE
-    macro_header: nl-types-macros.h
+    macro_header: nl_types-macros.h
 types:
   - type_name: nl_catd
 enums: []

libc/test/src/nl_types/nl_types_test.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "include/llvm-libc-macros/nl-types-macros.h"
+#include "include/llvm-libc-macros/nl_types-macros.h"
 #include "include/llvm-libc-types/nl_catd.h"
 #include "src/nl_types/catclose.h"
 #include "src/nl_types/catgets.h"