Introduce basic support for loading a precompiled preamble while

reparsing an ASTUnit. When saving a preamble, create a buffer larger
than the actual file we're working with but fill everything from the
end of the preamble to the end of the file with spaces (so the lexer
will quickly skip them). When we load the file, create a buffer of the
same size, filling it with the file and then spaces. Then, instruct
the lexer to start lexing after the preamble, therefore continuing the
parse from the spot where the preamble left off.

It's now possible to perform a simple preamble build + parse (+
reparse) with ASTUnit. However, one has to disable a bunch of checking
in the PCH reader to do so. That part isn't committed; it will likely
be handled with some other kind of flag (e.g., -fno-validate-pch).

As part of this, fix some issues with null termination of the memory
buffers created for the preamble; we were trying to explicitly
NULL-terminate them, even though they were also getting implicitly
NULL terminated, leading to excess warnings about NULL characters in
source files.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109445 91177308-0d34-0410-b5e6-96231b3b80d8

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