Fix recording preamble's conditional stack in skipped PP branches.

Summary:
This fixes PR34547.
`Lexer::LexEndOfFile` handles recording of ConditionalStack for
preamble and reporting errors about unmatched conditionalal PP
directives.
However, SkipExcludedConditionalBlock contianed duplicated logic for
reporting errors and clearing ConditionalStack, but not for preamble
recording.

This fix removes error reporting logic from
`SkipExcludedConditionalBlock`, unmatched PP conditionals are now
reported inside `Lexer::LexEndOfFile`.

Reviewers: erikjv, klimek, bkramer

Reviewed By: erikjv

Subscribers: nik, cfe-commits

Differential Revision: https://reviews.llvm.org/D37700

llvm-svn: 313014