[libc++] Remap headers in the debug info when building the library

The libc++ build includes a step where headers are generated. This is
required in order to preprocess some files such as the assertion handler
and the __config_site header. As a result, the library is built against
headers located inside the build directory, and the path to those headers
is what's included in the debug information of the library.

However, these headers in the build directory are usually not persistent,
which means that the debug information might end up referring to headers
that don't exist anymore. To solve this problem, this patch uses the
-fdebug-prefix-map flag supported by Clang and GCC to remap the generated
headers to the original headers in the source directory. This provides
the illusion that the library was truly built against the in-source
version of the headers.

Author: ldionne

libcxx/include/CMakeLists.txt

@@ -1706,6 +1706,11 @@ add_dependencies(cxx-headers generate-cxx-headers)
 target_include_directories(cxx-headers INTERFACE ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}
                                                  ${LIBCXX_GENERATED_INCLUDE_DIR})
 
+# Make sure to map the generated include directory back to libc++'s actual source directory when generating
+# debug information. Otherwise, the debug information will refer to generated headers which are created during
+# the build and generally not persistent.
+target_add_compile_flags_if_supported(cxx-headers INTERFACE "-fdebug-prefix-map=${LIBCXX_GENERATED_INCLUDE_DIR}=${LIBCXX_SOURCE_DIR}/include")
+
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
     get_filename_component(dir ${file} DIRECTORY)