change implicit int warnings to point to the identifier, not the

start of the declspec.  The fixit still goes there, and we underline
the declspec.  This helps when the start of the declspec came from a
macro that expanded from a system header.  For example, we now produce:

t.c:2:8: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
static x;
~~~~~~ ^

llvm-svn: 69777