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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