In the case several internal builtins defined by clang were not available,
ICU was replacing them by a stub implementation. But that was breaking
detection of availability of same methods in other parts of Chromium
(specifically in base/location.h).
Instead of that, this change creates ICU specific macros that will
map to those builtins when available, or to stub implementation if
not.
I.e. for the case of __has_builtin, previous implementation was
defining __has_builtin(x) as 0 in case it was not declared. With
new implementation, there is a macro UPRV_HAS_BUILTIN that maps
to __has_builtin if available, or 0 if not.
407442dec7