Adjust start-of-function braces to be compatible with Clang

gdb/testsuite/
	* gdb.cp/cpexprs.cc: Move braces to the same line as the start
	of the function to work across GCC and Clang.
	* gdb.cp/cpexprs.exp: Account for GCC/Clang difference in vtable
	pointer types (const void ** const V void **).
ada-lang.c: Expand standard_exc's introductory comment.

This patch expands standard_exc's introductory comment to explain
why this table does not include Numeric_Error.

gdb/ChangeLog:

        * ada-lang.c (standard_exc): Expand introductory comment.
Fix and XFAIL test due to GCC PR55641, passes with clang

gdb/testsuite/
	* gdb.python/lib-types.exp: Fix test and xfail under gcc due to gcc/55641.