Fix use-after-free found by address-san on -r322028.

r322028 attempted to remove something from the "Manglings"
list when it was no longer valid, and did so with 'erase'.

However, StringRefs to these were stored, so these became
dangling references.  This patch changes to using 'remove' instead
of 'erase' to keep the strings valid.

llvm-svn: 322052