Re-land r303274: "[CrashRecovery] Use SEH __try instead of VEH when available"

We have to check gCrashRecoveryEnabled before using __try.

In other words, SEH works too well and we ended up recovering from
crashes in implicit module builds that we weren't supposed to. Only
libclang is supposed to enable CrashRecoveryContext to allow implicit
module builds to crash.

llvm-svn: 303279