Copy string_view files from libstdc++

Copy string_view files from libstdc++

This patch copies the following files from libstdc++ (commit
02a4441f002c):

  ${gcc}/libstdc++-v3/include/experimental/string_view
    -> ${binutils-gdb}/gdb/common/gdb_string_view.h

  ${gcc}/libstdc++-v3/include/experimental/bits/string_view.tcc
    -> ${binutils-gdb}/gdb/common/gdb_string_view.tcc

The local modifications are done in the following patch in order to make
it easier to review them.

gdb/ChangeLog:

	* common/gdb_string_view.h: New file.
	* common/gdb_string_view.tcc: New file.