[DebugInfo][COFF] Fix reading variable-length encoded records

While reading Codeview records which contain variable-length encoded integers,
such as LF_BCLASS, LF_ENUMERATE, LF_MEMBER, LF_VBCLASS or LF_IVBCLASS,
the record's size would be improperly calculated in cases where the value was
indeed of a variable length (>= LF_NUMERIC). This caused a bad alignement on
the next record, which would/might crash later on.

Differential Revision: https://reviews.llvm.org/D45104

llvm-svn: 329659