Lex: reduce buffer size, add a test

Reduce the stack usage as hopefully include paths are usually not too long.  Add
a test case for the path normalisation behaviour.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203632 91177308-0d34-0410-b5e6-96231b3b80d8
Lexer: split test into separate files

Unfortunately, XFAIL applies to the entire file.  Split the test into two
different units, enabling the proper application of XFAIL on the test which is
expected to fail on Windows.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203633 91177308-0d34-0410-b5e6-96231b3b80d8