libstdc++: Reduce Doxygen output for PDF

Including the header source code in the doxygen-generated PDF file makes
it too large, and causes pdflatex to run out of memory. If we only set
SOURCE_BROWSER=YES for the HTML docs then we won't include the sources
in the PDF file.

There are several macros defined for std::valarray that are only used to
generate repetitive code and then #undef'd. Those aren't useful in the
doxygen docs, especially the ones that reuse the same name in different
files. Omitting them avoids warnings about duplicate labels in the
refman.tex file.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (SOURCE_BROWSER): Only set to YES for
	HTML docs.
	* include/bits/gslice_array.h (_DEFINE_VALARRAY_OPERATOR): Omit
	from doxygen docs.
	* include/bits/indirect_array.h (_DEFINE_VALARRAY_OPERATOR):
	Likewise.
	* include/bits/mask_array.h (_DEFINE_VALARRAY_OPERATOR):
	Likewise.
	* include/bits/slice_array.h (_DEFINE_VALARRAY_OPERATOR):
	Likewise.
	* include/std/valarray (_DEFINE_VALARRAY_UNARY_OPERATOR)
	(_DEFINE_VALARRAY_AUGMENTED_ASSIGNMENT)
	(_DEFINE_VALARRAY_EXPR_AUGMENTED_ASSIGNMENT)
	(_DEFINE_BINARY_OPERATOR): Likewise.

(cherry picked from commit afa69618)