[lld] Allow unrecognized signatures in debug sections

An unrecognized signature (magic) at the beginning of a debug section
should not be a fatal error; it only means that the debug information
is in a format that is not supported by LLD. This can be due to it
being in CodeView versions 3 or earlier. These can occur in old import
libraries from legacy SDKs.

The test case was verified to work with MS link.exe.

Patch by Vladimir Panteleev!

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D63177

llvm-svn: 363212