[Lex] Avoid out-of-bounds dereference in LexAngledStringLiteral.

Fix makes the loop in LexAngledStringLiteral more like the loops in
LexStringLiteral, LexCharConstant. When we skip a character after
backslash, we need to check if we reached the end of the file instead of
reading the next character unconditionally.

Discovered by OSS-Fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3832

rdar://problem/35572754

Reviewers: arphaman, kcc, rsmith, dexonsmith

Reviewed By: rsmith, dexonsmith

Subscribers: cfe-commits, rsmith, dexonsmith

Differential Revision: https://reviews.llvm.org/D41423

llvm-svn: 322390