PR17290: Use 'false' macro in fix-it hint for initializing a variable of type

_Bool in C, if the macro is defined. Also teach FixItUtils to look at whether
the macro was defined at the source location for which it is creating a fixit,
rather than looking at whether it's defined *now*. This is especially relevant
for analysis-based warnings which are delayed until end of TU.

llvm-svn: 191057