This fixes PR34547.
Lexer::LexEndOfFile handles recording of ConditionalStack for
preamble and reporting errors about unmatched conditionalal PP
However, SkipExcludedConditionalBlock contianed duplicated logic for
reporting errors and clearing ConditionalStack, but not for preamble
This fix removes error reporting logic from
SkipExcludedConditionalBlock, unmatched PP conditionals are now
reported inside Lexer::LexEndOfFile.