[BOLT] Fix data race in MCPlusBuilder::getOrCreateAnnotationIndex (#67004)

MCPlusBuilder::getOrCreateAnnotationIndex(Name) can be called from
different threads, for example when making use of
ParallelUtilities::runOnEachFunctionWithUniqueAllocId.

The race occurs when an Index for a particular annotation Name needs to
be created for the first time.

For example, this can easily happen when multiple "copies" of an
analysis pass run on different BinaryFunctions, and the analysis pass
creates a new Annotation Index to be able to store analysis results as
annotations.

This was found by using the ThreadSanitizer.

No regression test was added; I don't think there is good way to write
regression tests that verify the absence of data races?

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Co-authored-by: Amir Ayupov <fads93@gmail.com>