COFF: Cache the result of library searches.

File system operations were still dominating the profile on Windows. In this
case we were spending a significant amount of our time repeatedly searching
for libraries as a result of processing linker directives. Address this
by caching whether we have already found a library with a given name. For
chrome_child.dll:

Before: 10.53s
After: 6.88s

Differential Revision: https://reviews.llvm.org/D27840

llvm-svn: 289915