Address "always inline function is not always inlinable" warning with GCC.

Address "always inline function is not always inlinable" warning with GCC.

When an always_inline function is used prior to the functions definition,
the compiler may not be able to inline it as requested by the attribute.
GCC flags the `basic_string(CharT const*)` function as one such example.

This patch supresses the warning, and the problem, by moving the
definition of the string constructor to the inline declaration.
This ensures the body is available when it is first ODR used.

llvm-svn: 337235