diagnostics: Support for -finput-charset [PR93067]

Adds the logic to handle -finput-charset in layout_get_source_line(), so that
source lines are converted from their input encodings prior to being output by
diagnostics machinery. Also adds the ability to strip a UTF-8 BOM similarly.

gcc/c-family/ChangeLog:

	PR other/93067
	* c-opts.c (c_common_input_charset_cb): New function.
	(c_common_post_options): Call new function
	diagnostic_initialize_input_context().

gcc/d/ChangeLog:

	PR other/93067
	* d-lang.cc (d_input_charset_callback): New function.
	(d_init): Call new function
	diagnostic_initialize_input_context().

gcc/fortran/ChangeLog:

	PR other/93067
	* cpp.c (gfc_cpp_post_options): Call new function
	diagnostic_initialize_input_context().

gcc/ChangeLog:

	PR other/93067
	* coretypes.h (typedef diagnostic_input_charset_callback): Declare.
	* diagnostic.c (diagnostic_initialize_input_context): New function.
	* diagnostic.h (diagnostic_initialize_input_context): Declare.
	* input.c (default_charset_callback): New function.
	(file_cache::initialize_input_context): New function.
	(file_cache_slot::create): Added ability to convert the input
	according to the input context.
	(file_cache::file_cache): Initialize the new input context.
	(class file_cache_slot): Added new m_alloc_offset member.
	(file_cache_slot::file_cache_slot): Initialize the new member.
	(file_cache_slot::~file_cache_slot): Handle potentially offset buffer.
	(file_cache_slot::maybe_grow): Likewise.
	(file_cache_slot::needs_read_p): Handle NULL fp, which is now possible.
	(file_cache_slot::get_next_line): Likewise.
	* input.h (class file_cache): Added input context member.

libcpp/ChangeLog:

	PR other/93067
	* charset.c (init_iconv_desc): Adapt to permit PFILE argument to
	be NULL.
	(_cpp_convert_input): Likewise. Also move UTF-8 BOM logic to...
	(cpp_check_utf8_bom): ...here.  New function.
	(cpp_input_conversion_is_trivial): New function.
	* files.c (read_file_guts): Allow PFILE argument to be NULL.  Add
	INPUT_CHARSET argument as an alternate source of this information.
	(read_file): Pass the new argument to read_file_guts.
	(cpp_get_converted_source): New function.
	* include/cpplib.h (struct cpp_converted_source): Declare.
	(cpp_get_converted_source): Declare.
	(cpp_input_conversion_is_trivial): Declare.
	(cpp_check_utf8_bom): Declare.

gcc/testsuite/ChangeLog:

	PR other/93067
	* gcc.dg/diagnostic-input-charset-1.c: New test.
	* gcc.dg/diagnostic-input-utf8-bom.c: New test.

libcpp/include/cpplib.h

@@ -1379,6 +1379,20 @@ extern struct _cpp_file *cpp_get_file (cpp_buffer *);
 extern cpp_buffer *cpp_get_prev (cpp_buffer *);
 extern void cpp_clear_file_cache (cpp_reader *);
 
+/* cpp_get_converted_source returns the contents of the given file, as it exists
+   after cpplib has read it and converted it from the input charset to the
+   source charset.  Return struct will be zero-filled if the data could not be
+   read for any reason.  The data starts at the DATA pointer, but the TO_FREE
+   pointer is what should be passed to free(), as there may be an offset.  */
+struct cpp_converted_source
+{
+  char *to_free;
+  char *data;
+  size_t len;
+};
+cpp_converted_source cpp_get_converted_source (const char *fname,
+					       const char *input_charset);
+
 /* In pch.c */
 struct save_macro_data;
 extern int cpp_save_state (cpp_reader *, FILE *);
@@ -1449,6 +1463,7 @@ class cpp_display_width_computation {
 /* Convenience functions that are simple use cases for class
    cpp_display_width_computation.  Tab characters will be expanded to spaces
    as determined by TABSTOP.  */
+
 int cpp_byte_column_to_display_column (const char *data, int data_length,
 				       int column, int tabstop);
 inline int cpp_display_width (const char *data, int data_length,
@@ -1461,4 +1476,7 @@ int cpp_display_column_to_byte_column (const char *data, int data_length,
 				       int display_col, int tabstop);
 int cpp_wcwidth (cppchar_t c);
 
+bool cpp_input_conversion_is_trivial (const char *input_charset);
+int cpp_check_utf8_bom (const char *data, size_t data_length);
+
 #endif /* ! LIBCPP_CPPLIB_H */