Make meson recognize the Qualcomm LLVM toolchain

Meson calls `path/to/clang++ --version` to guess which build toolchain the
user has picked to build the source code. For the Qualcomm LLVM toolchain,
the output have an unusual output as shown below:

```
clang version 8.0.12
Snapdragon LLVM ARM Compiler 8.0.12 (based on llvm.org 7.0+)
Target: arm-unknown-linux-gnueabi
Thread model: posix
Repository: (ssh://git-hexagon-aus.qualcomm.com:...)
InstalledDir: /pkg/qct/software/llvm/release/arm/8.0.12/bin
```

Another unusual pattern is the output of `path/to/ld.qcld --version`:

```
ARM Linker from Snapdragon LLVM ARM Compiler Version 8.0.12
ARM Linker based on LLVM version: 8.0
```

The Meson logic is modified accordingly so that Meson can correctly
determine toolchain as "LLVM aarch64 cross-compiler on GNU/Linux64 OS".

This is the expected output of
`meson --native-file native_file.ini --cross-file cross_file.ini build/aarch64-debug/`:

```
...
C++ compiler for the host machine: ... (clang 8.0.12 "clang version 8.0.12")
C++ linker for the host machine: ... ld.lld 8.0.12
...
```

1 files changed, 6 insertions, 2 deletions

diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index 9830b45..bf75c80 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -55,6 +55,7 @@ from .linkers import (
     GnuBFDDynamicLinker,
     GnuGoldDynamicLinker,
     LLVMDynamicLinker,
+    QualcommLLVMDynamicLinker,
     MSVCDynamicLinker,
     OptlinkDynamicLinker,
     PGIDynamicLinker,
@@ -1028,10 +1029,13 @@ class Environment:
             check_args += override
 
         _, o, e = Popen_safe(compiler + check_args)
-        v = search_version(o)
+        v = search_version(o + e)
         if o.startswith('LLD'):
             linker = LLVMDynamicLinker(
                 compiler, for_machine, comp_class.LINKER_PREFIX, override, version=v)  # type: DynamicLinker
+        elif 'Snapdragon' in e and 'LLVM' in e:
+            linker = QualcommLLVMDynamicLinker(
+                compiler, for_machine, comp_class.LINKER_PREFIX, override, version=v)  # type: DynamicLinker
         elif e.startswith('lld-link: '):
             # The LLD MinGW frontend didn't respond to --version before version 9.0.0,
             # and produced an error message about failing to link (when no object
@@ -1227,7 +1231,7 @@ class Environment:
                 return cls(
                     compiler, version, for_machine, is_cross, info, exe_wrap,
                     target, linker=linker)
-            if 'clang' in out:
+            if 'clang' in out or 'Clang' in out:
                 linker = None
 
                 defines = self.get_clang_compiler_defines(compiler)