[ThinLTO]Clean up 'import-assume-unique-local' flag. (#102424)

While manual compiles can specify full file paths and build automation
tools use full, unique paths in practice, it's not clear whether it's a
general good practice to enforce full paths (fail a build if relative
paths are used).

`NumDefs == 1` condition [1] should hold true for many internal-linkage
vtables as long as full paths are indeed used to salvage the marginal
performance when local-linkage vtables are imported due to indirect
reference.
#100448 (comment)
has more details.

[1]
https://github.com/llvm/llvm-project/pull/100448/files#diff-e7cb370fee46f0f773f2b5429dfab36b75126d3909ae98ee87ff3d0e3f75c6e9R215

Author: mingmingl-llvm

compiler-rt/test/profile/Linux/instrprof-vtable-value-prof.cpp

@@ -141,7 +141,6 @@
 // RUN:    -g -flto=thin -fwhole-program-vtables -fno-split-lto-unit -O2 \
 // RUN:    -mllvm -enable-vtable-value-profiling -Wl,-mllvm,-enable-vtable-value-profiling \
 // RUN:    -mllvm -enable-vtable-profile-use \
-// RUN:    -Wl,-plugin-opt,-import-assume-unique-local \
 // RUN:    -Wl,-mllvm,-enable-vtable-profile-use -Rpass=pgo-icall-prom \
 // RUN:    -Wl,-mllvm,-print-after=pgo-icall-prom \
 // RUN:    -Wl,-mllvm,-filter-print-funcs=main lib.cpp main.cpp 2>&1 \

llvm/lib/Transforms/IPO/FunctionImport.cpp

@@ -174,17 +174,6 @@ static cl::opt<std::string> WorkloadDefinitions(
              "}"),
     cl::Hidden);
 
-static cl::opt<bool> ImportAssumeUniqueLocal(
-    "import-assume-unique-local", cl::init(false),
-    cl::desc(
-        "By default, a local-linkage global variable won't be imported in the "
-        "edge mod1:func -> mod2:local-var (from value profiles) since compiler "
-        "cannot assume mod2 is compiled with full path which gives local-var a "
-        "program-wide unique GUID. Set this option to true will help cross "
-        "module import of such variables. This is only safe if the compiler "
-        "user specify the full module path."),
-    cl::Hidden);
-
 static cl::opt<std::string>
     ContextualProfile("thinlto-pgo-ctx-prof",
                       cl::desc("Path to a contextual profile."), cl::Hidden);
@@ -214,9 +203,8 @@ static std::unique_ptr<Module> loadFile(const std::string &FileName,
 static bool shouldSkipLocalInAnotherModule(const GlobalValueSummary *RefSummary,
                                            size_t NumDefs,
                                            StringRef ImporterModule) {
-  // We can import a local from another module if all inputs are compiled
-  // with full paths or when there is one definition.
-  if (ImportAssumeUniqueLocal || NumDefs == 1)
+  // We can import a local when there is one definition.
+  if (NumDefs == 1)
     return false;
   // In other cases, make sure we import the copy in the caller's module if the
   // referenced value has local linkage. The only time a local variable can