libstdc++: Refactor implementation of operator+ for std::string

Until now operator+(char*, string) and operator+(string, char*) had
different performance characteristics. The former required a single
memory allocation and the latter required two. This patch makes the
performance equal.

After consultation with Jonathan, it seemed like a good idea to create a
single function that performed one-allocation string concatenation that
could be used by various different version of operator+. This patch adds
such a function and calls it from the relevant implementations.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (__str_cat): Add common function
	that performs single-allocation string concatenation.
	(operator+): Use __str_cat.
	* include/bits/basic_string.tcc (operator+): Move to .h and
	define inline using __str_cat.

Signed-off-by: Will Hawkins <whh8b@obs.cr>