clang-format: fix block OpeningLineIndex around preprocessor

Summary:
The current code would return an incorrect value when a preprocessor
directive is present immediately after the opening brace: this causes
the nanespace end comment fixer to break in some places, for exemple it
would not add the comment in this case:

  namespace a {
  #define FOO
  }

Fixing the computation is simple enough, but it was breaking a feature,
as it would cause comments to be added also when the namespace
declaration was dependant on conditional compilation.

To fix this, a hash of the current preprocessor stack/branches is
computed at the beginning of parseBlock(), so that we explicitely do not
store the OpeningLineIndex when the beginning and end of the block are
not in the same preprocessor conditions.

Tthe hash is computed based on the line, but this could propbably be
improved by using the actual condition, so that clang-format would be
able to match multiple identical #ifdef blocks.

Reviewers: krasimir, djasper

Reviewed By: krasimir

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D35483

llvm-svn: 309369