[LLD][COFF] Reset outputSections for successive runs

The global variable outputSections in the COFF writer was not
cleared between runs which caused successive calls to lld::coff::link
to generate invalid binaries. These binaries when loaded would result
in "invalid win32 applications" and/or "bad image" errors.

Differential Revision: https://reviews.llvm.org/D86401

Author: baszalmstra

lld/COFF/Writer.cpp

@@ -601,6 +601,9 @@ void Writer::finalizeAddresses() {
 void Writer::run() {
   ScopedTimer t1(codeLayoutTimer);
 
+  // First, clear the output sections from previous runs
+  outputSections.clear();
+
   createImportTables();
   createSections();
   createMiscChunks();