Defer source path remap tilde expansion until source file use

When reading source path remappings out of a dSYM, lldb currently
does tilde expansion -- expanding the tilde-username and checking
that the destination pathname exists, for each dSYM with the path
remappings.  This cost happens during lldb's initial process launch
/ load, an especially perf-sensitive time.  Inside Apple, we have
dSYMs with source path remappings pointing to NFS directories where
these extra stats for every dSYM can be very expensive if the network
is slow.

This patch instead keeps the source path mapping in the original
tilde-username terms and does the tilde expansion when we need
to read a specific source file from one of the modules.  We'll
be stat'ing all of those inodes to load the source file anyway,
so the fact that we do the tilde expansion on every source file
we load, it doesn't cost us significantly.

Differential Revision: https://reviews.llvm.org/D126435
rdar://77091379

(cherry picked from commit c274b6e5)