File::Read(), when asked to read the contents of a file into a heap

File::Read(), when asked to read the contents of a file into a heap
buffer and to add a nul terminator byte, was incorrectly resizing
its buffer so the nul terminator was not included.

Problem found by clang ASAN instrumentation when using an
expression prefix file which was read via this mechanism.

<rdar://problem/19556459>

llvm-svn: 226753