MCContext::reportError: don't call report_fatal_error

Errors from MCAssembler, MCObjectStreamer and *ObjectWriter typically cause a crash:

```
% cat c.c
int bar;
extern int foo __attribute__((alias("bar")));
% clang -c -fcommon c.c
fatal error: error in backend: Common symbol 'bar' cannot be used in assignment expr
PLEASE submit a bug report to ...
Stack dump:
...
```

`LLVMTargetMachine::addPassesToEmitFile` constructs `MachineModuleInfoWrapperPass`
which creates a MCContext without SourceMgr. `MCContext::reportError` calls
`report_fatal_error` which gets captured by Clang `LLVMErrorHandler` and gets translated
to the output above.

Since `MCContext::reportError` errors indicate user errors, such a crashing style error
is inappropriate. So this patch changes `report_fatal_error` to `SourceMgr().PrintMessage`.
```
% clang -c -fcommon c.c
<unknown>:0: error: Common symbol 'bar' cannot be used in assignment expr
```

Ideally we should at least recover the original filename (the line information
is generally lost).  That requires general improvement to MC diagnostics,
because currently in many cases SMLoc information is lost.

Author: MaskRay

llvm/lib/MC/MCContext.cpp

@@ -825,13 +825,13 @@ void MCContext::reportError(SMLoc Loc, const Twine &Msg) {
 
   // If we have a source manager use it. Otherwise, try using the inline source
   // manager.
-  // If that fails, use the generic report_fatal_error().
+  // If that fails, construct a temporary SourceMgr.
   if (SrcMgr)
     SrcMgr->PrintMessage(Loc, SourceMgr::DK_Error, Msg);
   else if (InlineSrcMgr)
     InlineSrcMgr->PrintMessage(Loc, SourceMgr::DK_Error, Msg);
   else
-    report_fatal_error(Msg, false);
+    SourceMgr().PrintMessage(Loc, SourceMgr::DK_Error, Msg);
 }
 
 void MCContext::reportWarning(SMLoc Loc, const Twine &Msg) {

llvm/test/CodeGen/AMDGPU/GlobalISel/lds-zero-initializer.ll

@@ -1,3 +1,3 @@
-; RUN: not --crash llc -global-isel -march=amdgcn -mcpu=tonga < %S/../lds-zero-initializer.ll 2>&1 | FileCheck %s
+; RUN: not llc -global-isel -march=amdgcn -mcpu=tonga < %S/../lds-zero-initializer.ll 2>&1 | FileCheck %s
 
-; CHECK: error: <unknown>:0:0: in function load_zeroinit_lds_global void (i32 addrspace(1)*, i1): unsupported initializer for address space
+; CHECK: <unknown>:0: error: lds: unsupported initializer for address space

llvm/test/CodeGen/AMDGPU/lds-initializer.ll

@@ -1,5 +1,5 @@
-; RUN: not --crash llc -march=amdgcn -mcpu=tahiti < %s 2>&1 | FileCheck %s
-; RUN: not --crash llc -march=amdgcn -mcpu=tonga < %s 2>&1 | FileCheck %s
+; RUN: llc -march=amdgcn -mcpu=tahiti < %s -o /dev/null 2>&1 | FileCheck %s
+; RUN: llc -march=amdgcn -mcpu=tonga < %s -o /dev/null 2>&1 | FileCheck %s
 
 ; CHECK: lds: unsupported initializer for address space
 

llvm/test/CodeGen/AMDGPU/lds-zero-initializer.ll

@@ -1,5 +1,5 @@
-; RUN: not --crash llc -march=amdgcn -mcpu=tahiti < %s 2>&1 | FileCheck %s
-; RUN: not --crash llc -march=amdgcn -mcpu=tonga < %s 2>&1 | FileCheck %s
+; RUN: llc -march=amdgcn -mcpu=tahiti < %s -o /dev/null 2>&1 | FileCheck %s
+; RUN: llc -march=amdgcn -mcpu=tonga < %s -o /dev/null 2>&1 | FileCheck %s
 
 ; CHECK: lds: unsupported initializer for address space
 

llvm/test/CodeGen/XCore/section-name.ll

@@ -1,9 +1,9 @@
-; RUN: not --crash llc < %s -march=xcore 2>&1 | FileCheck %s
+; RUN: llc < %s -march=xcore -o /dev/null 2>&1 | FileCheck %s
 
 @bar = internal global i32 zeroinitializer
 
 define void @".dp.bss"() {
   ret void
 }
 
-; CHECK: LLVM ERROR: invalid symbol redefinition
+; CHECK: <unknown>:0: error: invalid symbol redefinition