opt-viewer: Fix syntax highlighting

Syntax highlighting has been done line-at-a-time. Done this way, the lexer
resets the context at each line, distorting the formatting.

This change will render the whole file at once and feed the highlighted text
line-at-a-time to be wrapped by the SourceFileRenderer.

Leading/trailing newlines were being ignored by Pygments but since each line
was rendered in its own row, it didn't matter. This bug was masked by the
line-at-a-time algorithm. So now we need to add "stripnl=False" to the 
CppLexer to change its behavior to match the expectation.

llvm-svn: 295546