[PCH] Fixed preamble breaking with BOM presence (and particularly, fluctuating BOM presence)

This patch fixes broken preamble-skipping when the preamble region includes a byte order mark (BOM). Previously, parsing would fail if preamble PCH generation was enabled and a BOM was present.

This also fixes preamble invalidation when a BOM appears or disappears. This may seem to be an obscure edge case, but it happens regularly with IDEs that pass buffer overrides that never (or always) have a BOM, yet the underlying file from the initial parse that generated a PCH might (or might not) have a BOM.

I've included a test case for these scenarios.

Differential Revision: https://reviews.llvm.org/D37491

llvm-svn: 313796