libstdc++: Optimize operator+(string/char*, char*/string) equally

libstdc++: Optimize operator+(string/char*, char*/string) equally

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

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (operator+(const string&, const char*)):
	Remove naive implementation.
	* include/bits/basic_string.tcc (operator+(const string&, const char*)):
	Add single-allocation implementation.

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