The function `tryExpandAsInteger` attempts to extract an integer from a
macro definition. Previously, the attempt would fail when the macro is
from a PCH, because the function tried to access the text buffer of the
source file, which does not exist in case of PCHs. The fix uses
`Preprocessor::getSpelling`, which works in either cases.
rdar://151403070
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Co-authored-by: Balazs Benics <benicsbalazs@gmail.com>
