Make BinaryStreamReader::readCString a bit faster.
Previously it would do a character by character search for a null terminator, to account for the fact that an arbitrary stream need not store its data contiguously so you couldn't just do a memchr. However, the stream API has a function which will return the longest contiguous chunk without doing a copy, and by using this function we can do a memchr on the individual chunks. For certain types of streams like data from object files etc, this is guaranteed to find the null terminator with only a single memchr, but even with discontiguous streams such as MappedBlockStream, it's rare that any given string will cross a block boundary, so even those will almost always be satisfied with a single memchr. This optimization is worth a 10-12% reduction in link time (4.2 seconds -> 3.75 seconds) Differential Revision: https://reviews.llvm.org/D33503 llvm-svn: 303918
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