[ELF] Change the search order for dependent libraries

[phosek]

Summary

When processing dependent libraries, if there's a directory of the same
name as the library being searched for, either in the current directory
or earlier in the search order, LLD will try to open it and report an
error. This is because LLD uses file existence check. To address this
issue we reverse the order, searching the library by basename first
and only considering search paths later, and current directory last.

[MaskRay]

Thanks! There is some optimization we can make, e.g. if there is no path separator, don't bother with searchLibraryBaseName, but it may not justify the added code complexity.
@bd1976llvm would like to see smaller amount of code on this feature IIRC.

lld/ELF/InputFiles.cpp

 // section contains a list of library specifiers such as `m` for libm. This
 // function resolves a given name by finding the first matching library checking
 // the various ways that a library can be specified to LLD. This ELF extension
 // is a form of autolinking and is called `dependent libraries`. It is currently
 // unique to LLVM and lld.
 static void addDependentLibrary(StringRef specifier, const InputFile *f) {
   if (!config->dependentLibraries)
     return;
-  if (fs::exists(specifier))
-    driver->addFile(specifier, /*withLOption=*/false);
-  else if (Optional<std::string> s = findFromSearchPaths(specifier))
+  if (Optional<std::string> s = searchLibraryBaseName(specifier))
     driver->addFile(*s, /*withLOption=*/true);
-  else if (Optional<std::string> s = searchLibraryBaseName(specifier))
+  else if (Optional<std::string> s = findFromSearchPaths(specifier))
     driver->addFile(*s, /*withLOption=*/true);
+  else if (fs::exists(specifier))
+    driver->addFile(specifier, /*withLOption=*/false);
   else
     error(toString(f) +
           ": unable to find library from dependent library specifier: " +
           specifier);
 }
 
 // Record the membership of a section group so that in the garbage collection
 // pass, section group members are kept or discarded as a unit.

lld/test/ELF/deplibs.s

 # RUN: cp %t.dir/foo.a %t.cwd/libcmdline.a
 # RUN: ld.lld %t.o libcmdline.a -o /dev/null -L %t.dir --trace 2>&1 | \
 # RUN:   FileCheck %s -DOBJ=%t.o -DSO=%t.dir --check-prefix CMDLINE \
 # RUN:                --implicit-check-not=foo.a --implicit-check-not=libbar.so
 # CMDLINE:      [[OBJ]]
 # CMDLINE-NEXT: {{^libcmdline\.a}}
 
 ## LLD tries to resolve dependent library specifiers in the following order:
-##   1) The literal name in the current working directory.
-##   2) The literal name in a library search path.
-##   3) The name, prefixed with "lib" and suffixed with ".so" or ".a" in a
+##   1) The name, prefixed with "lib" and suffixed with ".so" or ".a" in a
 ##      library search path. This means that a directive of "foo.a" could lead
 ##      to a library named "libfoo.a.a" being linked in.
+##   2) The literal name in a library search path.
+##   3) The literal name in the current working directory.
 ## When using library search paths for dependent libraries, LLD follows the same
 ## rules as for libraries specified on the command line.
 # RUN: cp %t.dir/foo.a %t.cwd/foo.a
 # RUN: cp %t.dir/foo.a %t.dir/libfoo.a.a
+
 # RUN: ld.lld %t.o -o /dev/null -L %t.dir --trace 2>&1 | \
-# RUN:   FileCheck %s -DOBJ=%t.o -DDIR=%t.dir --check-prefix=CWD \
+# RUN:   FileCheck %s -DOBJ=%t.o -DDIR=%t.dir --check-prefix=LIBA-DIR \
 # RUN:                --implicit-check-not=foo.a --implicit-check-not=libbar.so
 
-# CWD:      [[OBJ]]
-# CWD-NEXT: {{^foo\.a}}
+# LIBA-DIR:      [[OBJ]]
+# LIBA-DIR-NEXT: [[DIR]]{{[\\/]}}libfoo.a.a
 
-# RUN: rm %t.cwd/foo.a
+# RUN: rm %t.dir/libfoo.a.a
 # RUN: ld.lld %t.o -o /dev/null -L %t.dir --trace 2>&1 | \
 # RUN:   FileCheck %s -DOBJ=%t.o -DDIR=%t.dir --check-prefix=PLAIN-DIR \
 # RUN:                --implicit-check-not=foo.a --implicit-check-not=libbar.so
 
 # PLAIN-DIR:      [[OBJ]]
 # PLAIN-DIR-NEXT: [[DIR]]{{[\\/]}}foo.a
 
 # RUN: rm %t.dir/foo.a
 # RUN: ld.lld %t.o -o /dev/null -L %t.dir --trace 2>&1 | \
-# RUN:   FileCheck %s -DOBJ=%t.o -DDIR=%t.dir --check-prefix=LIBA-DIR \
+# RUN:   FileCheck %s -DOBJ=%t.o -DDIR=%t.dir --check-prefix=CWD \
 # RUN:                --implicit-check-not=foo.a --implicit-check-not=libbar.so
 
-# LIBA-DIR:      [[OBJ]]
-# LIBA-DIR-NEXT: [[DIR]]{{[\\/]}}libfoo.a.a
+# CWD:      [[OBJ]]
+# CWD-NEXT: {{^foo\.a}}
 
     call foo
 .section ".deplibs","MS",@llvm_dependent_libraries,1
     .asciz "foo.a"
     ## Show that an unneeded archive must be present but may not be linked in.
     .asciz "bar"