Put bcache inside "namespace gdb"

Put bcache inside "namespace gdb"

This avoids a conflict with a system "struct bcache" on
Solaris (see e.g.
https://www.isi.edu/nsnam/archive/ns-users/webarch/2001/msg05393.html)

Note that the Solaris conflict for now only surfaces with
--enable-targets=all (which the build bot doesn't use).

gdb/ChangeLog:

2019-12-06  Christian Biesinger  <cbiesinger@google.com>

	* bcache.c: Put in namespace gdb.
	* bcache.h: Likewise.
	* gdbtypes.c (check_types_worklist): Update.
	(types_deeply_equal): Update.
	* macrotab.c (struct macro_table) <bcache>: Update.
	(new_macro_table): Update.
	* macrotab.h (struct bcache): Put this forward declaration
	inside namespace gdb.
	(new_macro_table): Update.
	* objfiles.h (struct objfile_per_bfd_storage) <filename_cache>:
	Update.
	<macro_cache>: Update.
	* psymtab.h: (psymtab_storage) <psymbol_cache>: Update.

Change-Id: I843d5e91f7ccb3db6d1099a8214c15a74510256f