[Support/CrashRecoveryContext] Make sure CrashRecoveryContext does not clear...

[Support/CrashRecoveryContext] Make sure CrashRecoveryContext does not clear the thread-local "CurrentContext"
in the "parent" thread, when we are using CrashRecoveryContext::RunSafelyOnThread.

When using CrashRecoveryContext::RunSafelyOnThread, we would set a CrashRecoveryContextImpl* to a thread-local variable
for the "child" thread, but CrashRecoveryContext would erroneously clear it in the "parent" thread.

The result was that if CrashRecoveryContext::RunSafelyOnThread was called again in the "child" thread it would mess up
crash-recovery for its parent.

A test for this will be added in the clang repository.
rdar://14204560

llvm-svn: 184380