stop defining _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS when …

…building libc++abi

This was needed because libc++'s <exception> header omits
set_unexpected() (which libcxxabi defines) if that define isn't
set -- but <exception> also doesn't omit it if _LIBCPP_BUILDING_LIBRARY
is defined, which it is after
https://chromium-review.googlesource.com/c/chromium/src/+/3469341

No behavior change.

Bug: 1298070
Change-Id: I6b289656dc9bbc3a5428ac288bda54755bcf221d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3472583
Commit-Queue: Nico Weber <[email protected]>
Auto-Submit: Nico Weber <[email protected]>
Reviewed-by: Arthur Eubanks <[email protected]>
Commit-Queue: Arthur Eubanks <[email protected]>
Cr-Commit-Position: refs/heads/main@{#972605}

buildtools/third_party/libc++abi/BUILD.gn

@@ -59,7 +59,6 @@ source_set("libc++abi") {
 
   defines = [
     "LIBCXXABI_SILENT_TERMINATE",
-    "_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS",
 
     # TODO(crbug.com/1298070) _LIBCPP_CONSTINIT is defined in a libc++ revision
     # more recent than the one currently used in Chromium.