Add 'inline' attribute to __init to inline the basic_string's constructor

basic_string's constructor calls init which was not getting inlined.  This
prevented optimization of const string as init would appear as a call in between
a string's def and use.

Patch by Laxman Sole and Aditya Kumar.

Differential Revision: https://reviews.llvm.org/D22782

llvm-svn: 278356